they are only limited in terms of the materials used to make

they are only limited in terms of the materials used to make them.--make them后面省略掉什么?
This assumption misses a significant difference between the two disciplines.Fine-art objects are not constrained by the laws of physics in the same way that applied-art objects are.Because their primary purpose is not functional,they are only limited in terms of the materials used to make them.请认真帮我回答?