"The identification of the code system that defines the meaning of the symbol in the code." - HL7 FHIR, Coding.system Note that in FHIR, Coding.system is a URI pointing to the code system (which presumes that each code system has a URI, and that each has a separate URI for each version of the code system). In the FHIM, we've made this a separate class to show how the logical pieces of a code system identifier would be made up.