The time argument can have different formats. First, it can be an absolute time in the format hh:mm, in which hh is the hour (1 or 2 digits) and mm is the minute of the hour (in two digits). Second, it can be in the format +m, in which m is the number of minutes to wait. The word now is an alias for +0.
The -f flag means `reboot fast'. This only creates an advisory file /fastboot which can be tested by the system when it comes up again. The boot rc file can test if this file is present, and decide not to run fsck(1) since the system has been shut down in the proper way. After that, the boot process should remove /fastboot.
The -n flag causes shutdown not to call init, but to kill all running processes itself. After all processes are killed, shutdown will try to run the script /etc/rc.d/rc.halt fast (or rc.reboot). It also tests for the presence of the equivalent files rc.0 and rc.6. If this script returns or can't be executed (because it's not present for example) shutdown will turn off quota and accounting, turn off swapping and unmount all filesystems.
shutdown: no authorized users logged in
to the (physical) system console. The format of /etc/shutdown.allow is one user name per line. Empty lines and comment lines (prefixed by a #) are allowed. Currently there is a limit of 32 users in this file.
/fastboot /etc/inittab /etc/rc.d/rc.halt /etc/rc.d/rc.reboot /etc/shutdown.allow