The Clinical Guidelines IG marks many properties as "Must Support" in its profiles. FHIR indicates that IGs should define what "Must Support" means – for the whole IG, for a profile, or for a specific element (see: http://hl7.org/fhir/R4/profiling.html#mustsupport). I could not find any guidance regarding "Must Support" in this IG, but that would be essential for an implementor to know before claiming conformance to the IG.