At each level in the Entity-Relationship Model, one Attribute shall be defined as a Unique Key. A single value in a Unique Key Attribute shall uniquely identify a single entity at a given level. That is, two entities at the same level may not have the same Unique Key value.
C-FIND, C-MOVE, and C-GET SCPs shall support existence and matching of all Unique Keys defined by a Query/Retrieve Information Model. All entities managed by C-FIND, C-MOVE, and C-GET SCPs shall have a specific non-zero length Unique Key value.
Unique Keys may be contained in the Identifier of a C-FIND request. Unique Keys shall be contained in the Identifier of C-MOVE and C-GET requests.