IMS network uses following identifiers to identify an user in IMS network.
IMPI (IP Multimedia Private Identity) : is a unique permanently allocated global identity assigned by home network operator, used for registration, authorization, administration and accounting purposes. Every IMS user shall have one IMPI.
IMPU (IP Multimedia Public Identity) : is used by an user for requesting communications to other users. There can be multiple IMPUs per IMPI. IMPU can also be shared with another phone.
GRUU (Globally Rout-able User Agent URI) : is an identity that identifies a unique combination of IMPU and UE instance.