In the context of HL7 FHIR (Fast Healthcare Interoperability Resources), the terms "id" and "identifier" refer to specific elements used for identifying resources within the FHIR data model. For a newbie, these terms can be confusingly similar, but they serve distinct purposes.
Look at the below Patient resource for August T. Faulkner:
The resource has an id of “1” — generated by the FHIR server when the resource was created.
Patient August T. Faulkner also has a identifier (Medical Record Number) — possibly provided by the hospital — of 78510398960