The intro says: "This guide and the US Core profiles have become the foundation for US Realm FHIR implementation guides. This annual release reflects changes to U.S. Core Data for Interoperability (USCDI) v3 and comments and requests from the US Realm FHIR community."  However, the Clinical Notes section requires that to be compliant with the US core document reference profile you need to support at least 5 specific note types.  In addition, several diagnostic service types.  While this is excellent for the certified EHR use case, it is not foundational to other types of US use cases that are not certified EHR centric.  One specific example is Advance Directives in which the server may be a registry of Advance Directives.  In that case the only applicable document type would be advance directive.  It seems that "not applicable" would make sense as an option.  This would allow the Advance Directive US Realm guide to build on US Core.  It feels like US Core needs a spec on top to represent the EHR specific use cases and that US Core should provide more of a foundation so as to support a variety of use cases would open up more opportunities.