Sessreg is a simple program for managing utmp/wtmp entries for xdm sessions.
System V has a better interface to /etc/utmp than BSD; it dynamically allocates entries in the file, instead of writing them at fixed positions indexed by position in /etc/ttys.
To manage BSD-style utmp files, sessreg has two strategies. In conjunction with xdm, the -x option counts the number of lines in /etc/ttys and then adds to that the number of the line in the Xservers file which specifies the display. The display name must be specified as the "line-name" using the -l option. This sum is used as the "slot-number" in /etc/utmp that this entry will be written at. In the more general case, the -s option specifies the slot-number directly. If for some strange reason your system uses a file other that /etc/ttys to manage init, the -t option can direct sessreg to look elsewhere for a count of terminal sessions.
Conversely, System V managers will not ever need to use these options (-x, -s and -t). To make the program easier to document and explain, sessreg accepts the BSD-specific flags in the System V environment and ignores them.
BSD and Linux also have a host-name field in the utmp file which doesn't exist in System V. This option is also ignored by the System V version of sessreg.