In the FHIR standard, every resource has a unique identifier for that resource instance. According to the FHIR specification: “Each resource has an "id" element which contains the logical identity of the resource assigned by the server responsible for storing it.”. This identifier will differ if the resource is moved or persisted on another server (it is analogous to the primary key in a database). Typically, a resource will also have one or more “business” identifiers. These have a business meaning outside the FHIR server (e.g. an NHS number or Organisation Identifier), and can be used in resources held on multiple servers (each instance of which would have different logical IDs to identify them). See: