FHIR has two id-shaped fields on almost every resource, and they do different jobs. Resource.id is the server's logical id — where the resource lives. Resource.identifier[] is the business identity — the MRN, order number, or external...

Every FHIR resource has a logical id. It can be a UUID, a sequential integer, a natural business key, or anything else that satisfies the id datatype rules. The choice is not academic. A resource id is what every reference in the graph...