What does the Law of Concentration of Matter not apply to?

The Law of Concentration of Matter, also known as the Law of Conservation of Mass, does not apply to nuclear reactions, where mass can be converted into energy as described by Einstein's equation (E=mc^2). It also does not apply in processes involving antimatter, where matter and antimatter annihilate each other, resulting in a loss of mass. Additionally, in certain quantum processes at very small scales, the traditional interpretation of mass conservation may not hold.