This is unclear:

If the itemExtractionContext is empty, then the Questionnaire is being used to create a resource. If the itemExtractionContext has a resource (or set of resources), then the Questionnaire is being used to update the resource(s).

See Zulip chat: https://chat.fhir.org/#narrow/stream/179255-questionnaire/topic/extensions.20for.20new.20resource.20with.20definition.20based.20extraction