This case-sensitive code system http://fhir.org/guides/who/core/CodeSystem/who-identifier-type defines the following codes:

Code	Display	Definition
client-assigned	Client Assigned	An identifier for the resource that was generated as part of creating the resource in a client application. The identifier captures the logical id of the resource when it was created, and is typically expected to be a globally unique identifier such as a UUID.
server-assigned	Server Assigned	An identifier for the resource that was assigned by a server as part of creating the resource in a create or upsert interaction. The identifier captures the logical id of the resource when it was created, though this may be used by the client as well as part of a negotiated id assignment.