This is the CodeSystem that contains all the codes in Health Insurance Prospective Payment System (HIPPS) (https://www.cms.gov/Medicare/Medicare-Fee-for-Service-Payment/ProspMedicareFeeSvcPmtGen/HIPPSCodes) and Health Insurance Prospective Payment System (HIPPS) (https://www.cms.gov/Medicare/Medicare-Fee-for-Service-Payment/ProspMedicareFeeSvcPmtGen/HIPPSCodes). E.g. what you have to deal with if you get resources containing codes in either of them