Question: What is the required format for the username part of an External Identifier, which identifies a subscription associated with an IMSI (International Mobile Subscriber Identity)?
Option 1: The username part must contain a Local Identifier, as specified in telecom standards for packet data networks and applications.
Option 2: The username part must be a duly registered Internet domain name, ensuring global uniqueness.
Option 3: The username part is freely assignable by the user, with no specific format requirements.
Option 4: The username part must match the IMSI exactly, for direct association.
Explanation: According to telecom standards, the username part of an External Identifier requires a Local Identifier for structured identification in communications with packet data networks and applications, ensuring organization and uniqueness in subscriber identification.