It's ERC721, and cubes are non-fungible. I made it so the contract token ID corresponds to the cube ID directly, as a little nice-to-have (it was really just a big pet peeve when it wasn't). That's a really good question: not once did I consider that cubes could be fungible; I'd always assumed that they'd have the intrinsic property of a (x,y,z) location tied to their ID. I'm reachable at esher@projectcube.io :)