hdmi av cable

can someone explain exactly what the hdmi av cable for xbox does.I think it allows you to play 1080p. But it would also let me put my 360 onto my surround sound correct?? But I still could listen to the xbox through the tv speakers correct because of the hdmi to the tv and the audio to the surround sound.

oh yeah it also lets you use an optical wire for your surround systemmarcssc

what does this mean??