In his blog, Parts and ownership, Alexander R. Pruss asserts that matter receives its identity from the substance that it makes up:

“Therefore, no bit of matter can be a part of two substances. But everything that exists is a substance or a mode, relation, trope, accident or the like. A proper part of a substance is not a mode, relation, trope, accident or the like. Hence, if a substance has a proper part, that proper part is a substance. But the matter of a proper part A of a substance B would then be a part of both substance A and substance B, which contradicts the thesis that a bit of matter can’t be a part of two substances.“

If a bit of matter x is part of substance A and a bit of matter y is part of substance B, then if A is distinct from B, it follows that x is distinct from y.