killall is used by shutdown(1M) to kill all active processes not directly related to the shutdown procedure. killall terminates all processes with open files s

killall is used by shutdown(1M) to kill all active processes not directly related to the shutdown procedure. killall terminates all processes with open files so that the mounted file systems will be unbusied and can be unmounted. killall sends signal (see kill(1)) to the active processes. If no signal is specified, a default of 15 is used. Try and guess where I got that info from.