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