Each method is designed so two IDs both generated with that method don't collide with each other. Reserving bits for the method avoids the problem of having to

Each method is designed so two IDs both generated with that method don't collide with each other. Reserving bits for the method avoids the problem of having to also ensure that for all pair-wise combinations of methods, two IDs generated with those methods don't collide. This is also why there is only a finite set of universally agreed upon methods.