The documentation for the following items in the core specification:

• Code System Supplements section for the CodeSystem resource type; and
• The synonym (to be renamed alternateCode — see FHIR-39542) code system concept property included in
  • the defined concept properties section of the CodeSystem resource type; and
  • the http://hl7.org/fhir/concept-properties CodeSystem resource

needs to be updated to explicitly indicate whether alternate codes can be provided as properties in FHIR CodeSystem supplements, and if so, under what circumstances/conditions.