Question: During IMS Registration, how does an S-CSCF determine the default Public User Identity from an implicitly registered Public User Identity set?
Option 1: It selects the first non-barred Public User Identity in the set received from the user information download.
Option 2: It always chooses the last Public User Identity listed in the implicitly registered set.
Option 3: The default is predetermined by the network and not derived from the implicitly registered set.
Option 4: All Public User Identities in the set are equally considered default, with no single identity prioritized.
Explanation: According to IMS registration procedures, when a user's Public User Identity is registered, it implies the registration of its associated implicitly registered Public User Identity set. The S-CSCF identifies the first non-barred Public User Identity within this set and designates it as the default to ensure a distinct and usable identity for the user, preventing conflicts or service disruptions.