Question: In a 3GPP system, what is a key characteristic of a User Identifier provided for a user?
Option 1: It is dependent on existing subscription identifiers (e.g., IMSI, MSISDN) and device identifiers (e.g., IMEI).
Option 2: It is independent of existing identifiers relating to subscription or device and can be provided by the operator's network or a 3rd party.
Option 3: It requires the user to always access the network via their Home PLMN (HPLMN) for authentication.
Option 4: It cannot be used for non-3GPP devices connected to the network via a UE acting as a gateway.
Explanation: The correct answer highlights a crucial aspect of User Identifiers in the 3GPP system: their independence from other identifiers (such as IMSI, MSISDN for subscriptions, and IMEI for devices) and their potential provision by either the operator's network or a third-party entity. This characteristic ensures flexibility and security in user identification across different access technologies and devices.