Add View Definition non-goals to main page.

input/pagecontent/index.md

@@ -64,6 +64,31 @@ Such tabular views can be created for any FHIR resource, with
 [View Definition page](StructureDefinition-ViewDefinition.html) for the full definition
 of the above structure.
 
+### View Definition non-goals
+View Definitions are intentionally constrained to a narrow set of functionality to make them easily
+and broadly implementable, while deferring higher-level capabilities to database engines or processing
+pipelines that solve those problems well. Therefore it's important to know what View Definitions
+do *not* do, by design:
+
+#### A single View Definition will not join different resources in any way
+A single ViewDefinition defines a tabular view of exactly one resource type, like a view of `Patient`
+or a view of `Condition` resources. Any joins between resources are exclusively in downstream systems,
+like between database tables computed by view definitions. This makes it possible for a wide set of
+FHIR infrastructure to implement this spec, and lets database engines or processing pipelines join as
+needed.
+
+#### View Definitions do not have sorting, aggregation, or limit capabilities
+View Definitions define only the logical schema of views, and therefore defer sorting, aggergation or limit
+operations to engines, along with cross-view joins. *View Runners* (described below) or future FHIR
+server operations may accept limits or sort columns as part of their operations, so users at runtime can
+specify what they need dynamically and independently of the definition of the view itself.
+
+#### View Definitions are not aware of output formats
+ViewDefinitions themselves are independnet of any tech stack and therefore unaware of the output format.
+*View Runners* are the component that applies definitions to a particular stack, producing output like
+a database table, Parquet file, CSV, or another format specific to the runner.
+
+
 ### System Layers
 
 The [View Definition](StructureDefinition-ViewDefinition.html) is the central element of this