Which of the following is the conventional purpose of Linux UIDs that are lower than 100?

Only UID 0 is reserved for the superuser (root); other low UIDs are for various system services, not for superusers.

System admin accounts typically refer to human administrators and are usually assigned UIDs above the system account range (e.g., 1000+).

UIDs in the lower range, typically 0-99 (though the upper bound can vary by distribution, e.g., 499 or 999), are conventionally reserved for system accounts used by services, daemons, and other non-interactive processes, with UID 0 specifically for the root superuser.

The low UID range is primarily for system account assignment, not specifically left unused due to exploits, although system accounts can be targets of exploits.

UIDs identify users, and while users belong to groups, the low UID range's primary purpose is not for matching with GIDs in grouping users.