Question: In a Mission Critical (MC) service utilizing SIP signalling control plane, what is the primary purpose of a private user identity when a signalling user agent sends registration requests to the registrar/application service selection?
Option 1: To enable routing of signalling messages through the SIP system
Option 2: To authenticate the signalling user agent by providing corresponding credentials to the registrar/application service selection
Option 3: To associate application-level attributes of MC services (e.g., MCPTT ID) with the user
Option 4: To uniquely identify MC service UEs when multiple devices or users share the same device
Explanation: The private user identity is used for authentication purposes, allowing the registrar/application service selection to find and verify the corresponding credentials of the signalling user agent, similar to the IMPI's role in secure IMS authentication.