I believe that it just refers to the tank volume even though that is not the total amount of water you are filtering with you live rock. My guess is based on the fact that if you had a lot of water that was not in your display, and you added all of that rock to your main display, there would hardly be any water in the tank. Also if you plan to run a Berlin style sump then some of that live rock can go in there.
Goodluck.