CM Mom is exactly right. When you board, you can make ressie for one brunch and one dinner per cabin. People do "cheat" by travelling with another family and each stateroom makes a ressie for 4.
The idea is for everyone to have a fair chance at a reservation. If you'd like to go again, you can call on the day you'd like to go. If there are any cancellations, they will put you in then.