The top (and other personalized objects) tell the people whether they're in a dream or not. Each object (token) behaves in a way that is known only by the user. For example, Ellen Page's token, the chess piece, has been hollowed out, but she's the only one that knows that. If Ellen ever discovers that her pawn is actually a full one (normal, not hollow), that means she's in someone else's dream, and they have incorrectly re-created her token.