VHA Enterprise Reference Terminology is based on CHI standard terminologies (e.g., SNOMED-CT, LOINC, HL7, NDF-RT, etc.) when available and on VHA own code sets when necessary (e.g., allergens). All concepts used within the VHA clinical environment receive a VHA Unique IDentifier or VUID. VHA Enterprise Reference Terminology complies with the semantics of the HL7 message structure