diff --git a/guide/release-notes/v3.0.16.adoc b/guide/release-notes/v3.0.16.adoc
index 3ddde663..cfa211fa 100644
--- a/guide/release-notes/v3.0.16.adoc
+++ b/guide/release-notes/v3.0.16.adoc
@@ -4,6 +4,7 @@ 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
 
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/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>