Need more of an explanation as to what this is for.  The "data enterer" responsible for resources within the message doesn't belong on the MessageHeader because the header will be discarded.  In general, enterers belong either on the resources or on Provenance associated with those resources.  If there's going to be an 'enterer' on the MessageHeader, it should be associated with the header itself.  So provide guidance that enterer should only be used when the MessageHeader represents a user-initiated event and there isn't a resource in the payload representing that event.  For example, a user-initiated query or operation invocation.  It would NOT be appropriate to have a message seeking fulfillment of a prescription and using MessageHeader.enterer to reflect the data enterer of the MedicaitonRequest.

Please clarify the element by improving the definition or adding usage notes.