The value is be unique within the defined system and have a consistent meaning wherever it appears. Identifier.system is always case sensitive. Identifier.value is to be treated as case sensitive unless knowledge of the Identifier.system allows the processer to be confident that non-case-sensitive processing is safe.
The system is a URI (Uniform Resource Identifier )that defines a set of identifiers (i.e. how the value is made unique). It might be a specific application or a recognized standard/specification for a set of identifiers or a way of making identifiers unique. If the system is a URL (Uniform Resource Locator), it should resolve. Resolution might be to a web page that describes the identifier system and/or supports look-up of identifiers. Alternatively, it could be to a Naming System resource instance.
Fast Healthcare Interoperability Resources (FHIR) is a draft standard describing data formats and elements (known as “resources”) and an application programming interface (API) for exchanging electronic health records. The standard was created by the Health Level Seven International (HL7) health-care standards organization.
Its goal is to facilitate interoperation between legacy healthcare systems, to make it easy to provide healthcare information to healthcare providers and individuals on a wide variety of devices from computers to tablets to cell phones, and to allow third-party application developers to provide medical applications which can be easily integrated into existing systems.
FHIR provides an alternative to document-centric approaches by directly exposing discrete data elements as services. For example, basic elements of healthcare like patients, admissions, diagnostic reports and medications can each be retrieved and manipulated via their own resource URLs (Uniform Resource Locators). FHIR was supported at an American Medical Informatics Association meeting by many EHR (Electronic Health Record) vendors which value its open and extensible nature.