There is no requirement that the requester knows the FHIR Patient.id in advance of the exchange.  The FHIR Patient.id is solicited via one of two methods  1) query for patient by .identifier (using an MRN or Member ID) or  2) Performing the Patient Match operation and providing sufficient demographics to match to a single patient Each of the above will return a FHIR Patient.id for a successful query/match. Expand the documentation to clarify the above process Add that the source system SHOULD support the patient match operation.