Security flags (or access flags) are like keys which the sysop can choose to give to specific users or groups of users and then require a user to possess one or more flags to access specific areas or features of the BBS (like placing a lock or locks on specific areas or features).
Flags have no pre-defined meaning or usage and are available solely for the definition of the sysop.
There are 4 flag groups (flags sets 1, 2, 3, and 4) and within each flag set are 26 flags identified with the alphabetic characters A through Z, for a total of 104 flags. So a specific flag in a specific group is usually referred to like FLAG 1A, meaning flag 'A' from set #1.
For convenience, a sysop can modify their text/menu/flags*.asc file(s) to document which flags have been allocated for what security purposes. These menu files can be displayed in the online user editor.