Independent draw order and relations makes it possible to do things as a man sitting on a horse or flying through a window. To sit on a horse, add a draw order change where the horse is put between the legs of the man. Then add a relation from the man to the horse. So when you move the horse with matrix tool, the man is following.
You can also change the relation from one container to another directly. This is used in cases as when a person gives a letter to another person.