I'm not familiar with "AC" but "ACD" stands for automatice closure device. This is standard on newer Legends and the first stage automatically closes off when the air pressure is removed, supposedly preventing contaminants from getting in.
I suggest getting a secondary air source or octo similar to your primary. For example, the Legend has an octo which has a yellow face plate on it and the guts are the same as the primary Legend second stage (I believe).
You'll also find that Legends come as the Legend, a Legend LX (able to adjust the breathing capability), and the Supreme that can apply to the Legend and the Legend LX. Supreme usually designates primarily set up for cold water diving with a lip shield, lower IP setting and a silver face plate instead of the normal gold one.
I have a Legend and an Legend LX and can honestly say I have never used the LX adjustable feature.
I suggest going to the aqualung website to get details on any of these, I'm sure they can tell you more than I can.