Espalier is the ancient technique of training a tree to grow flat against a support such as a trellis, wall or wires. The art of espalier is mainly used to save space by maintaining the width and height of the tree whilst reducing the depth. It also looks AMAZING!
You can definitely use potted plants for espalier, keep in mind though that once your apple tree is trained on its frame it is very hard to move. Choose a pot that gives your apple room to grow, generally your pot should be at least three times bigger than the original pot it came in. Little Apple Co apples are dwarf varieties which make them perfect for pots!