We will define 'mustSupport' in CRD as follows: For CRD clients, if the client maintains the data element and surfaces it to users, then it SHALL be exposed in their FHIR interface when the data exists and privacy constraints permit. For CRD services, the server SHALL leverage mustSupport elements as available and appropriate to provide decision support. NOTE: These requirements are somewhat different from US-Core and HRex because the implementation needs are different.  In US-Core, there is generally an expectation for clients to modify code and persistence layers to add support mustSupport elements for supported profiles.  This expectation does not hold for CRD.  However, CRD does require surfacing elements in the FHIR interface if the system maintains the element.