An User Identifier is a persistent, non-reassigned identifier.
An Identity Provider (or Attribute Authority) is said to release a User Identifier when it releases at least one of the following attributes on the wire:
A Service Provider is said to request a Non-Private User Identifier when it requests the
eduPersonUniqueId attribute in metadata or a query. Alternatively, a Service Provider may request a Non-Private User Identifier directly, as shown in the following example.