Login user IDs
The operating system can identify users by their login user ID.
The login user ID allows the system to trace all user actions to their source. After a user logs in to the system and before the initial user program is run, the system sets the login ID of the process to the user ID found in the user database. All subsequent processes during the login session are tagged with this ID. These tags provide a trail of all activities performed by the login user ID.
You can reset the effective user ID, real user ID, effective group ID, real group ID, and supplementary group ID during the session, but you cannot change the login user ID.