Will add some documentation about the output expectations of $extract. Observation-based will always produce a 'transaction' Bundle, even if it's only creating or updating a single Observation Definition-based will always produce a 'transaction' Bundle that indicates what resources are to be created or updated.  The resources within the transaction can be anything (including Bundle resources such as FHIR documents) StructureMap-based may produce either a 'transaction' Bundle or a single resource instance (though the instance might itself be a different type of Bundle, such as a document or collection).  It is possible for StructureMaps to map to non-FHIR data structures, however there's no "built-in" mechanism to serialize such models, so additional information would need to be passed into the $extract process to guide such serialization.  Support for non-FHIR extracts is outside the scope of this specification.