Many of the profiles in this guide reference other FHIR resources that are also US Core Profiles. This is defined in the formal profile definitions. For example, US Core CareTeam Profile references US Core Patient. For any other references to base FHIR resources^1^ or not formally defined in a US Core Profiles, the referenced resource SHOULD be a US Core Profile if a US Core Profile exists for the resource type. 

Thank you for adding this. Please make sure the validator folks read it!