Identifier provides the ability to create a globally unique identifier.   This could be GUID a UUID or any other string that an organization that distinctly identifies the specific object.

Ideally, for the identifier to be globally unique it must have a system provided identifier and the system must be identified using a global naming authority. System identification uses the domain naming system (DNS) to assure they are globally unique and should be an URL. The system provided ID will typically be a primary or surrogate key in a database.

The URL, system and company attributes can be omitted only when the system context can be implied by a parent or ancestor element.