This is the CodeSystem that contains all the codes in code system properties mode (http://hl7.org/fhir/codesystem-properties-mode) and code system properties mode code system (http://hl7.org/fhir/codesystem-properties-mode). E.g. what you have to deal with if you get resources containing codes in either of them
Generated Narrative: CodeSystem 51c29a49-01b4-4d32-a0ac-08324781d817-20
This code system http://hl7.org/fhir/comparison/CodeSystem/51c29a49-01b4-4d32-a0ac-08324781d817-20
defines codes, but no codes are represented here