This is the CodeSystem that contains all the codes in European Petroleum Survey Group Geodetic Parameter Dataset Coordinate Reference System (http://terminology.hl7.org/CodeSystem/epsg-crs) and European Petroleum Survey Group Geodetic Parameter Dataset Coordinate Reference System (http://terminology.hl7.org/CodeSystem/epsg-crs). E.g. what you have to deal with if you get resources containing codes in either of them