Explanation
In a RESTful Application Programming (RAP) application, you can bind a CDS view to create a value help in the following objects:
Data model view: A data model view is a CDS view that defines the data structure and the associations of an entity in the RAP application. You can use the annotation @Consumption.valueHelpDefinition to bind a value help provider CDS view to an element of the data model view. The value help provider CDS view must contain the key fields of the value help entity and the fields that are displayed in the value help dialog. The value help annotation specifies the entity name, the element name, and optionally the additional binding conditions for the value help provider1.
Metadata Extension: A metadata extension is a CDS view that extends the metadata of another CDS view without changing its data structure. You can use the annotation @MetadataExtension.extendView to specify the target CDS view that you want to extend. You can then use the same annotation
@Consumption.valueHelpDefinition to bind a value help provider CDS view to an element of the target CDS view. The metadata extension allows you to add value help definitions to existing CDS views without modifying them2.
Projection View: A projection view is a CDS view that defines the projection of another CDS view. You can use the annotation @AbapCatalog.sqlViewType: #PROJECTION to specify that the CDS view is a projection view. You can then use the same annotation @Consumption.valueHelpDefinition to bind a value help provider CDS view to an element of the projection view. The projection view allows you to add value help definitions to projected elements of another CDS view3.
You cannot bind a value help provider CDS view to a behavior definition or a service definition, because these objects do not define the data structure or the metadata of an entity in the RAP application. A behavior definition defines the behavior and the validation rules of an entity, such as the create, read, update, and delete (CRUD) operations, the draft handling, the authorization checks, and the side effects4. A service definition defines the service exposure and the service binding of an entity, such as the protocol, the version, the namespace, and the service name5.
References: 1: Value Help with Additional Binding | SAP Help Portal 2: Metadata Extensions - ABAP Keyword Documentation 3: Projection Views - ABAP Keyword Documentation 4: Behavior Definition - ABAP Keyword Documentation 5: Service Definition - ABAP Keyword Documentation