Union of http://terminology.hl7.org/CodeSystem/GCRT and http://terminology.hl7.org/CodeSystem/GCRT
This is the CodeSystem that contains all the codes in GCRT (http://terminology.hl7.org/CodeSystem/GCRT) and GCRT (http://terminology.hl7.org/CodeSystem/GCRT). E.g. what you have to deal with if you get resources containing codes in either of them