Mutes all sounding notes that were turned on by received Note On messages, and which haven't yet been turned off by respective Note Off messages. This message is not supposed to mute any notes that the musician is playing on the local keyboard. So, if a device can't distinguish between notes played via its MIDI IN and notes played on the local keyboard, it should not implement All Sound Off.
Note: The difference between this message and All Notes Off is that this message immediately mutes all sound on the device regardless of whether the Hold Pedal is on, and mutes the sound quickly regardless of any lengthy VCA release times. It's often used by sequencers to quickly mute all sound when the musician presses "Stop" in the middle of a song.
The value byte isn't used and defaults to 0.