I found from googling that each letter stands for these:

unRaw (take control of keyboard back from X), tErminate (send SIGTERM to all processes, allowing them to terminate gracefully), kIll (send SIGKILL to all processes, forcing them to terminate immediately), Sync (flush data to disk), Unmount (remount all filesystems read-only), reBoot.

So if what B does is reboot, what difference does this make against just holding down the power key.

And what's the significance of other letters (I don't understand the terms).