The GRUU identifies a unique combination of Public User Identity and an instance of the UE. This allows the UE to address a SIP request to a specific combination of the Public User Identity and a registered UE. This prevents the request from getting forked to another UE registered for the IMPU. If the UICC is changed, the UE instance is still considered to be the same, so the UE instance ID is dependent on the device and not the UICC.
GRUUs can either be permanent (P-GRUU) or temporary (T-GRUU). A permanent GRUU, or Pub-GRUU, exposes the public user identity and remains in duration across multiple sessions. A T-GRUU, or Temporary GRUU, isis issued for the duration of a registration and does not expose public user identity. The IMS network can generate the GRUUs. The P-GRUU will always be the same combination for a particular UE instance. The T-GRUU combination will change with each registration.