diff --git a/guide/release-notes/main.adoc b/guide/release-notes/main.adoc
index b5a646f8..41d5e6cb 100644
--- a/guide/release-notes/main.adoc
+++ b/guide/release-notes/main.adoc
@@ -15,6 +15,8 @@ IMPORTANT: Changes in code lists are also reflected by changes in the validation
 
 :leveloffset: +1
 
+include::v3.0.16.adoc[]
+
 include::v3.0.15.adoc[]
 
 include::v3.0.14.adoc[]
diff --git a/guide/release-notes/v3.0.16.adoc b/guide/release-notes/v3.0.16.adoc
new file mode 100644
index 00000000..cfa211fa
--- /dev/null
+++ b/guide/release-notes/v3.0.16.adoc
@@ -0,0 +1,16 @@
+= Version 3.0.16
+[horizontal]
+Release date:: 2023-11-01
+
+== Changes to BIS document
+
+* A recommendation has been added to the Sales Order Reference element for handling cases where there is no Purchase Order Reference. This clarifies what to do with the mandatory UBL element for Purchase Order Reference. Example file is added which illustrates this.
+
+== Changes to code lists and validation artefacts
+
+* The validation rule identified as PEPPOL-EN16931-R006, which ensures that only one Invoice Object is allowed at the document level in an invoice/credit note, has been removed. This is because it duplicates the European standard rule UBL-SR-04, making it redundant and unnecessary.
+
+* The structure of the Peppol Schematron rule set has been improved for better quality. A function that was previously located in the Greek rules section is now correctly positioned alongside other functions.
+
+==  Changes to Country-Specific Rules
+
diff --git a/rules/examples/sales-order-example.xml b/rules/examples/sales-order-example.xml
new file mode 100644
index 00000000..87f16242
--- /dev/null
+++ b/rules/examples/sales-order-example.xml
@@ -0,0 +1,214 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<Invoice xmlns:cac="urn:oasis:names:specification:ubl:schema:xsd:CommonAggregateComponents-2"
+    xmlns:cbc="urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2"
+    xmlns="urn:oasis:names:specification:ubl:schema:xsd:Invoice-2">
+    <cbc:CustomizationID>urn:cen.eu:en16931:2017#compliant#urn:fdc:peppol.eu:2017:poacc:billing:3.0</cbc:CustomizationID>
+    <cbc:ProfileID>urn:fdc:peppol.eu:2017:poacc:billing:01:1.0</cbc:ProfileID>
+    <cbc:ID>Snippet1</cbc:ID>
+    <cbc:IssueDate>2017-11-13</cbc:IssueDate>
+    <cbc:DueDate>2017-12-01</cbc:DueDate>
+    <cbc:InvoiceTypeCode>380</cbc:InvoiceTypeCode>
+    <cbc:DocumentCurrencyCode>EUR</cbc:DocumentCurrencyCode>
+    <cbc:AccountingCost>4025:123:4343</cbc:AccountingCost>
+    <cbc:BuyerReference>0150abc</cbc:BuyerReference>
+	<cac:OrderReference>
+		<cbc:ID>NA</cbc:ID><!-- NA can be provided if there is no purchase order and only a sales order, as the chc:ID element is mandatory -->
+		<cbc:SalesOrderID>123456</cbc:SalesOrderID>
+	</cac:OrderReference>	
+    <cac:AccountingSupplierParty>
+        <cac:Party>
+            <cbc:EndpointID schemeID="0088">9482348239847239874</cbc:EndpointID>
+            <cac:PartyIdentification>
+                <cbc:ID>99887766</cbc:ID>
+            </cac:PartyIdentification>
+            <cac:PartyName>
+                <cbc:Name>SupplierTradingName Ltd.</cbc:Name>
+            </cac:PartyName>
+            <cac:PostalAddress>
+                <cbc:StreetName>Main street 1</cbc:StreetName>
+                <cbc:AdditionalStreetName>Postbox 123</cbc:AdditionalStreetName>
+                <cbc:CityName>London</cbc:CityName>
+                <cbc:PostalZone>GB 123 EW</cbc:PostalZone>
+                <cac:Country>
+                    <cbc:IdentificationCode>GB</cbc:IdentificationCode>
+                </cac:Country>
+            </cac:PostalAddress>
+            <cac:PartyTaxScheme>
+                <cbc:CompanyID>GB1232434</cbc:CompanyID>
+                <cac:TaxScheme>
+                    <cbc:ID>VAT</cbc:ID>
+                </cac:TaxScheme>
+            </cac:PartyTaxScheme>
+            <cac:PartyLegalEntity>
+                <cbc:RegistrationName>SupplierOfficialName Ltd</cbc:RegistrationName>
+                <cbc:CompanyID>GB983294</cbc:CompanyID>
+            </cac:PartyLegalEntity>
+        </cac:Party>
+    </cac:AccountingSupplierParty>
+    <cac:AccountingCustomerParty>
+        <cac:Party>
+            <cbc:EndpointID schemeID="0002">FR23342</cbc:EndpointID>
+            <cac:PartyIdentification>
+                <cbc:ID schemeID="0002">FR23342</cbc:ID>
+            </cac:PartyIdentification>
+            <cac:PartyName>
+                <cbc:Name>BuyerTradingName AS</cbc:Name>
+            </cac:PartyName>
+            <cac:PostalAddress>
+                <cbc:StreetName>Hovedgatan 32</cbc:StreetName>
+                <cbc:AdditionalStreetName>Po box 878</cbc:AdditionalStreetName>
+                <cbc:CityName>Stockholm</cbc:CityName>
+                <cbc:PostalZone>456 34</cbc:PostalZone>
+                <cac:Country>
+                    <cbc:IdentificationCode>SE</cbc:IdentificationCode>
+                </cac:Country>
+            </cac:PostalAddress>
+            <cac:PartyTaxScheme>
+                <cbc:CompanyID>SE4598375937</cbc:CompanyID>
+                <cac:TaxScheme>
+                    <cbc:ID>VAT</cbc:ID>
+                </cac:TaxScheme>
+            </cac:PartyTaxScheme>
+            <cac:PartyLegalEntity>
+                <cbc:RegistrationName>Buyer Official Name</cbc:RegistrationName>
+                <cbc:CompanyID schemeID="0183">39937423947</cbc:CompanyID>
+            </cac:PartyLegalEntity>
+            <cac:Contact>
+                <cbc:Name>Lisa Johnson</cbc:Name>
+                <cbc:Telephone>23434234</cbc:Telephone>
+                <cbc:ElectronicMail>lj@buyer.se</cbc:ElectronicMail>
+            </cac:Contact>
+        </cac:Party>
+    </cac:AccountingCustomerParty>
+    <cac:Delivery>
+        <cbc:ActualDeliveryDate>2017-11-01</cbc:ActualDeliveryDate>
+        <cac:DeliveryLocation>
+            <cbc:ID schemeID="0088">9483759475923478</cbc:ID>
+            <cac:Address>
+                <cbc:StreetName>Delivery street 2</cbc:StreetName>
+                <cbc:AdditionalStreetName>Building 56</cbc:AdditionalStreetName>
+                <cbc:CityName>Stockholm</cbc:CityName>
+                <cbc:PostalZone>21234</cbc:PostalZone>
+                <cac:Country>
+                    <cbc:IdentificationCode>SE</cbc:IdentificationCode>
+                </cac:Country>
+            </cac:Address>
+        </cac:DeliveryLocation>
+        <cac:DeliveryParty>
+            <cac:PartyName>
+                <cbc:Name>Delivery party Name</cbc:Name>
+            </cac:PartyName>
+        </cac:DeliveryParty>
+    </cac:Delivery>
+    <cac:PaymentMeans>
+        <cbc:PaymentMeansCode name="Credit transfer">30</cbc:PaymentMeansCode>
+        <cbc:PaymentID>Snippet1</cbc:PaymentID>
+        <cac:PayeeFinancialAccount>
+            <cbc:ID>IBAN32423940</cbc:ID>
+            <cbc:Name>AccountName</cbc:Name>
+            <cac:FinancialInstitutionBranch>
+                <cbc:ID>BIC324098</cbc:ID>
+            </cac:FinancialInstitutionBranch>
+        </cac:PayeeFinancialAccount>
+    </cac:PaymentMeans>
+    <cac:PaymentTerms>
+        <cbc:Note>Payment within 10 days, 2% discount</cbc:Note>
+    </cac:PaymentTerms>
+        <cac:AllowanceCharge>
+            <cbc:ChargeIndicator>true</cbc:ChargeIndicator>
+            <cbc:AllowanceChargeReason>Insurance</cbc:AllowanceChargeReason>
+            <cbc:Amount currencyID="EUR">25</cbc:Amount>
+            <cac:TaxCategory>
+                <cbc:ID>S</cbc:ID>
+                <cbc:Percent>25.0</cbc:Percent>
+                <cac:TaxScheme>
+                    <cbc:ID>VAT</cbc:ID>
+                </cac:TaxScheme>
+            </cac:TaxCategory>
+        </cac:AllowanceCharge>
+    <cac:TaxTotal>
+        <cbc:TaxAmount currencyID="EUR">331.25</cbc:TaxAmount>
+        <cac:TaxSubtotal>
+            <cbc:TaxableAmount currencyID="EUR">1325</cbc:TaxableAmount>
+            <cbc:TaxAmount currencyID="EUR">331.25</cbc:TaxAmount>
+            <cac:TaxCategory>
+                <cbc:ID>S</cbc:ID>
+                <cbc:Percent>25.0</cbc:Percent>
+                <cac:TaxScheme>
+                    <cbc:ID>VAT</cbc:ID>
+                </cac:TaxScheme>
+            </cac:TaxCategory>
+        </cac:TaxSubtotal>
+    </cac:TaxTotal>
+    <cac:LegalMonetaryTotal>
+        <cbc:LineExtensionAmount currencyID="EUR">1300</cbc:LineExtensionAmount>
+        <cbc:TaxExclusiveAmount currencyID="EUR">1325</cbc:TaxExclusiveAmount>
+        <cbc:TaxInclusiveAmount currencyID="EUR">1656.25</cbc:TaxInclusiveAmount>
+        <cbc:ChargeTotalAmount currencyID="EUR">25</cbc:ChargeTotalAmount>
+        <cbc:PayableAmount currencyID="EUR">1656.25</cbc:PayableAmount>
+    </cac:LegalMonetaryTotal>
+    
+<cac:InvoiceLine>
+        <cbc:ID>1</cbc:ID>
+    <cbc:InvoicedQuantity unitCode="DAY">7</cbc:InvoicedQuantity>
+    <cbc:LineExtensionAmount currencyID= "EUR">2800</cbc:LineExtensionAmount>
+        <cbc:AccountingCost>Konteringsstreng</cbc:AccountingCost>
+       <cac:OrderLineReference>
+            <cbc:LineID>123</cbc:LineID>
+        </cac:OrderLineReference>
+    <cac:Item>
+            <cbc:Description>Description of item</cbc:Description>
+            <cbc:Name>item name</cbc:Name>
+            <cac:StandardItemIdentification>
+                <cbc:ID schemeID="0088">21382183120983</cbc:ID>
+            </cac:StandardItemIdentification>
+            <cac:OriginCountry>
+                <cbc:IdentificationCode>NO</cbc:IdentificationCode>
+            </cac:OriginCountry>
+            <cac:CommodityClassification>
+                <cbc:ItemClassificationCode listID="SRV">09348023</cbc:ItemClassificationCode>
+            </cac:CommodityClassification>
+            <cac:ClassifiedTaxCategory>
+                <cbc:ID>S</cbc:ID>
+                <cbc:Percent>25.0</cbc:Percent>
+                <cac:TaxScheme>
+                    <cbc:ID>VAT</cbc:ID>
+                </cac:TaxScheme>
+            </cac:ClassifiedTaxCategory>
+        </cac:Item>
+    <cac:Price>
+        <cbc:PriceAmount currencyID="EUR">400</cbc:PriceAmount>
+    </cac:Price>
+    </cac:InvoiceLine>
+<cac:InvoiceLine>
+    <cbc:ID>2</cbc:ID>
+    <cbc:InvoicedQuantity unitCode="DAY">-3</cbc:InvoicedQuantity>
+    <cbc:LineExtensionAmount currencyID="EUR">-1500</cbc:LineExtensionAmount>
+    <cac:OrderLineReference>
+        <cbc:LineID>123</cbc:LineID>
+    </cac:OrderLineReference>
+    <cac:Item>
+        <cbc:Description>Description 2</cbc:Description>
+        <cbc:Name>item name 2</cbc:Name>
+        <cac:StandardItemIdentification>
+            <cbc:ID schemeID="0088">21382183120983</cbc:ID>
+        </cac:StandardItemIdentification>
+        <cac:OriginCountry>
+            <cbc:IdentificationCode>NO</cbc:IdentificationCode>
+        </cac:OriginCountry>
+        <cac:CommodityClassification>
+            <cbc:ItemClassificationCode listID="SRV">09348023</cbc:ItemClassificationCode>
+        </cac:CommodityClassification>
+        <cac:ClassifiedTaxCategory>
+            <cbc:ID>S</cbc:ID>
+            <cbc:Percent>25.0</cbc:Percent>
+            <cac:TaxScheme>
+                <cbc:ID>VAT</cbc:ID>
+            </cac:TaxScheme>
+        </cac:ClassifiedTaxCategory>
+    </cac:Item>
+    <cac:Price>
+        <cbc:PriceAmount currencyID="EUR">500</cbc:PriceAmount>
+    </cac:Price>
+</cac:InvoiceLine>
+</Invoice>
diff --git a/rules/sch/PEPPOL-EN16931-UBL.sch b/rules/sch/PEPPOL-EN16931-UBL.sch
index bef742f5..a65604fb 100644
--- a/rules/sch/PEPPOL-EN16931-UBL.sch
+++ b/rules/sch/PEPPOL-EN16931-UBL.sch
@@ -178,6 +178,24 @@ Last update: 2023 May release 3.0.15.
 ((string-to-codepoints(substring($val,11,1)) - 48) * 19)) mod 89 = 0
 "/>
   </function>
+
+  <!-- Functions and variable for Greek Rules -->
+  <function xmlns="http://www.w3.org/1999/XSL/Transform" name="u:TinVerification" as="xs:boolean">
+    <param name="val" as="xs:string"/>
+    <variable name="digits" select="
+			for $ch in string-to-codepoints($val)
+			return codepoints-to-string($ch)"/>
+    <variable name="checksum" select="
+			(number($digits[8])*2) +
+			(number($digits[7])*4) +
+			(number($digits[6])*8) +
+			(number($digits[5])*16) +
+			(number($digits[4])*32) +
+			(number($digits[3])*64) +
+			(number($digits[2])*128) +
+			(number($digits[1])*256) "/>
+    <value-of select="($checksum  mod 11) mod 10 = number($digits[9])"/>
+  </function>  
   <!-- Empty elements -->
   <pattern>
     <rule context="//*[not(*) and not(normalize-space())]">
@@ -213,7 +231,6 @@ Last update: 2023 May release 3.0.15.
       <assert id="PEPPOL-EN16931-R053" test="count(cac:TaxTotal[cac:TaxSubtotal]) = 1" flag="fatal">Only one tax total with tax subtotals MUST be provided.</assert>
       <assert id="PEPPOL-EN16931-R054" test="count(cac:TaxTotal[not(cac:TaxSubtotal)]) = (if (cbc:TaxCurrencyCode) then 1 else 0)" flag="fatal">Only one tax total without tax subtotals MUST be provided when tax currency code is provided.</assert>
       <assert id="PEPPOL-EN16931-R055" test="not(cbc:TaxCurrencyCode) or (cac:TaxTotal/cbc:TaxAmount[@currencyID=normalize-space(../../cbc:TaxCurrencyCode)] &lt;= 0 and cac:TaxTotal/cbc:TaxAmount[@currencyID=normalize-space(../../cbc:DocumentCurrencyCode)] &lt;= 0) or (cac:TaxTotal/cbc:TaxAmount[@currencyID=normalize-space(../../cbc:TaxCurrencyCode)] &gt;= 0 and cac:TaxTotal/cbc:TaxAmount[@currencyID=normalize-space(../../cbc:DocumentCurrencyCode)] &gt;= 0) " flag="fatal">Invoice total VAT amount and Invoice total VAT amount in accounting currency MUST have the same operational sign</assert>
-      <assert id="PEPPOL-EN16931-R006" test="(count(cac:AdditionalDocumentReference[cbc:DocumentTypeCode='130']) &lt;= 1)" flag="fatal">Only one invoiced object is allowed on document level</assert>
     </rule>
     <rule context="cbc:TaxCurrencyCode">
       <assert id="PEPPOL-EN16931-R005" test="not(normalize-space(text()) = normalize-space(../cbc:DocumentCurrencyCode/text()))" flag="fatal">VAT accounting currency code MUST be different from invoice currency code when provided.</assert>
@@ -477,23 +494,7 @@ Last update: 2023 May release 3.0.15.
     </rule>
   </pattern>
   <!-- GREECE -->
-  <!-- General functions and variable for Greek Rules -->
-  <function xmlns="http://www.w3.org/1999/XSL/Transform" name="u:TinVerification" as="xs:boolean">
-    <param name="val" as="xs:string"/>
-    <variable name="digits" select="
-			for $ch in string-to-codepoints($val)
-			return codepoints-to-string($ch)"/>
-    <variable name="checksum" select="
-			(number($digits[8])*2) +
-			(number($digits[7])*4) +
-			(number($digits[6])*8) +
-			(number($digits[5])*16) +
-			(number($digits[4])*32) +
-			(number($digits[3])*64) +
-			(number($digits[2])*128) +
-			(number($digits[1])*256) "/>
-    <value-of select="($checksum  mod 11) mod 10 = number($digits[9])"/>
-  </function>
+  <!-- General variable for Greek Rules -->
   <let name="isGreekSender" value="($supplierCountry ='GR') or ($supplierCountry ='EL')"/>
   <let name="isGreekReceiver" value="($customerCountry ='GR') or ($customerCountry ='EL')"/>
   <let name="isGreekSenderandReceiver" value="$isGreekSender and $isGreekReceiver"/>
diff --git a/rules/unit-UBL-PEPPOL/PEPPOL-EN16931-R006.xml b/rules/unit-UBL-PEPPOL/PEPPOL-EN16931-R006.xml
deleted file mode 100644
index 2b08c332..00000000
--- a/rules/unit-UBL-PEPPOL/PEPPOL-EN16931-R006.xml
+++ /dev/null
@@ -1,84 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<testSet xmlns="http://difi.no/xsd/vefa/validator/1.0" configuration="peppolbis-en16931-base-3.0-ubl">
-	<assert>
-    	<description>Verify existence of not more than a single object identifier on document level</description>
-		<scope>PEPPOL-EN16931-R006</scope>
-	</assert>
-	<test>
-		<assert>
-			<success>PEPPOL-EN16931-R006</success>
-		</assert>
-		<Invoice xmlns:cac="urn:oasis:names:specification:ubl:schema:xsd:CommonAggregateComponents-2"
-			xmlns:cbc="urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2"
-			xmlns="urn:oasis:names:specification:ubl:schema:xsd:Invoice-2">
-
-			<cac:AdditionalDocumentReference>
-				<cbc:ID schemeID="ABT">DR35141</cbc:ID>
-				<cbc:DocumentTypeCode>130</cbc:DocumentTypeCode>
-			</cac:AdditionalDocumentReference>
-			
-			<cac:AdditionalDocumentReference>
-				<cbc:ID>ts12345</cbc:ID>
-				<cbc:DocumentDescription>Technical specification</cbc:DocumentDescription>
-				<cac:Attachment>
-					<cac:ExternalReference>
-						<cbc:URI>www.techspec.no</cbc:URI>
-					</cac:ExternalReference>
-				</cac:Attachment>
-			</cac:AdditionalDocumentReference>
-
-		</Invoice>
-	</test>
-	<test>
-		<assert>
-			<success>PEPPOL-EN16931-R006</success>
-		</assert>
-		<Invoice xmlns:cac="urn:oasis:names:specification:ubl:schema:xsd:CommonAggregateComponents-2"
-			xmlns:cbc="urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2"
-			xmlns="urn:oasis:names:specification:ubl:schema:xsd:Invoice-2">
-
-			<cac:AdditionalDocumentReference>
-				<cbc:ID>ts12345</cbc:ID>
-				<cbc:DocumentDescription>Technical specification</cbc:DocumentDescription>
-				<cac:Attachment>
-					<cac:ExternalReference>
-						<cbc:URI>www.techspec.no</cbc:URI>
-					</cac:ExternalReference>
-				</cac:Attachment>
-			</cac:AdditionalDocumentReference>
-
-		</Invoice>
-	</test>
-	<test>
-		<assert>
-			<success>PEPPOL-EN16931-R006</success>
-		</assert>
-		<Invoice xmlns:cac="urn:oasis:names:specification:ubl:schema:xsd:CommonAggregateComponents-2"
-			xmlns:cbc="urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2"
-			xmlns="urn:oasis:names:specification:ubl:schema:xsd:Invoice-2">
-			
-<!--			<cac:AdditionalDocumentReference/>-->
-			
-			
-		</Invoice>
-	</test>
-	<test>
-		<assert>
-			<error>PEPPOL-EN16931-R006</error>
-		</assert>
-		<Invoice xmlns:cac="urn:oasis:names:specification:ubl:schema:xsd:CommonAggregateComponents-2"
-			xmlns:cbc="urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2"
-			xmlns="urn:oasis:names:specification:ubl:schema:xsd:Invoice-2">
-
-
-			<cac:AdditionalDocumentReference>
-				<cbc:DocumentTypeCode>130</cbc:DocumentTypeCode>
-			</cac:AdditionalDocumentReference>
-			
-			<cac:AdditionalDocumentReference>
-				<cbc:DocumentTypeCode>130</cbc:DocumentTypeCode>
-			</cac:AdditionalDocumentReference>
-
-		</Invoice>
-	</test>
-</testSet>
diff --git a/structure/syntax/ubl-creditnote.xml b/structure/syntax/ubl-creditnote.xml
index 2cbd0178..7614b31e 100755
--- a/structure/syntax/ubl-creditnote.xml
+++ b/structure/syntax/ubl-creditnote.xml
@@ -218,7 +218,7 @@
             <Element cardinality="0..1">
                 <Term>cbc:SalesOrderID</Term>
                 <Name>Sales order reference</Name>
-                <Description>An identifier of a referenced sales order, issued by the Seller.</Description>
+                <Description>An identifier of a referenced sales order, issued by the Seller. In cases where Sales order reference is provided, but there's no Purchase order reference, then set cac:OrderReference/cbc:ID to value "NA" as this element is mandatory in UBL.</Description>
                 <DataType>Document Reference</DataType>
                 <Reference type="BUSINESS_TERM">BT-14</Reference>
                 <Value type="EXAMPLE">112233</Value>
diff --git a/structure/syntax/ubl-invoice.xml b/structure/syntax/ubl-invoice.xml
index f95379b4..fd380a03 100755
--- a/structure/syntax/ubl-invoice.xml
+++ b/structure/syntax/ubl-invoice.xml
@@ -224,7 +224,7 @@
             <Element cardinality="0..1">
                 <Term>cbc:SalesOrderID</Term>
                 <Name>Sales order reference</Name>
-                <Description>An identifier of a referenced sales order, issued by the Seller.</Description>
+                <Description>An identifier of a referenced sales order, issued by the Seller. In cases where Sales order reference is provided, but there's no Purchase order reference, then set cac:OrderReference/cbc:ID to value "NA" as this element is mandatory in UBL.</Description>
                 <DataType>Document Reference</DataType>
                 <Reference type="BUSINESS_TERM">BT-14</Reference>
                 <Value type="EXAMPLE">112233</Value>