HL7 v2, FHIR, CDA, integration engines, profiles and extensions.
3 questions in Healthcare & Life Sciences.
FHIR is a resource-oriented REST API with a defined set of resources, constrained for real use by profiles and implementation guides. HL7 v2 is the older pipe-delimited messaging standard that still carries most of the event traffic inside hospitals, so the two coexist and an integration engine translates between them.
A code means nothing without its code system, its version and a mapping the receiver trusts. Local catalogues dominate, LOINC, SNOMED CT and ICD answer different questions, value set expansions drift with versions, and every local-to-standard map is lossy, owned and perpetually going stale.
Because a standard constrains structure and leaves the meaning local. Optionality, local identifiers and code lists, custom extensions and different workflow triggers mean conformance to the standard is not compatibility - which is why the fix is your own profile plus a test suite, not more mapping.