An ATW pattern us used to string the crosses from the top of the racket to the bottom of the racket when the mains end at the throat. When using an ATW pattern you will either use the long or short side to string one of the mains on the opposite side of the racket so you can string the racket crosses from top to bottom.
There are some that consider a box pattern an ATW pattern. This pattern is used on modern rackets when you want to string the racket so there are no overlapping strings on the outside of the frame. There are also some rackets that require this type of stringing like the Wilson T2000.
Anytime you use a box or ATW pattern you are going to be changing from mains to crosses and vice-versa. You will also be putting a lot of pressure on a small section of the frame.
Best option 99.44% of the time is to string the racket two piece.
Irvin