We will be removing the guidance on population of CDA identifiers entirely - it has no place in a FHIR-centric specification.  Our preference is not to exchange CDA documents at all.  If they are exchanged, it should be because the system is already producing CDA documents (and presumably knows how to do so).  This page will be refactored to cover the identifier expectations in FHIR and any impact there is in transporting CDA documents.