See https://jira.hl7.org/browse/FHIR-34990   Based on research, there are variations on the resources used to define subscriber through FHIR IGs. In PCT, we intend to leave subscriber as US Core Patient to remain compliant with Da Vinci HRex for STU1 unless there is a compelling reason not to at this time. Will remove MS from Coverage.subscriber, defaulting to what is in HRex, since all that is needed is subscriberId.  Note that the definition of Patient includes "other health-related services" as being in scope, and being a subscriber fits that scope.  Also, since it is unlikely that DOB and gender info will be available for the subscriber when not the patient, will add guidance to omit Patient.birthDate (it is 0..1 in US Core Patient and ok to omit when not available) and to use the code "unknown" for Patient.gender and document that the receiving system should not use that code for subscriber matching purposes (if unknown, don't try to match on gender).