The existence of a CodeSystem resource on a server declares the properties of the code system itself, as documented in the resource sections on Scope and Usage, Boundaries and Relationships and Background and Context. There is no mention in those sections regarding any expectation of whether the presence of a CodeSystem resource on a server also implies that the server has actually implemented and supports that particular code system and therefore is expected to behave accordingly in regard to the performance of any terminology service operations.

Discussion has occurred suggesting that querying the CodeSystem resource endpoint may be a useful means of determining which code systems are supported by a FHIR terminology service, but based on RESTful API principles, no such expectation should be presumed or implied. Instead, any expectations of server support and behavior in regard to a particular code system should be returned either in a server capability statement or preferably in a terminology service capability statement from a capabilities interaction. The terminology service capabilities interaction would be similar to GET [base]/metadata for the server capability statement (but with the specific interaction method presumably yet to be defined?).