Comment:

All FHIR resources that are intended to be persisted must be able to be usefully interpreted and actioned on a stand-alone basis without other resources being available. For example, a "pre-determination" or "pre-authorization" might be sent to a downstream system to support an eventual claim or a payor might wish to provide an un-requested pre-authorization for a service it had algorithmically determined was appropriate/needed. Whether this is current practice or not, "event" resources should never pre-suppose the existence of a request. The only exception in FHIR is non-persisted (throw-away) responses to operations such as OperationOutcomes or Parameters.

Summary:

Response' doesn't allow for resources to stand alone