IO-UF division

Immovable Object - Unstoppable force division, or IO-UF division, is the classification theory of almost every universe in the Pigverse. It states that in order to avoid paradoxes, any universe within the Pigverse can only contain either an immovable object or an unstoppable force.

IO-universes
IO-universes can contain immovable objects, but can't contain unstoppable forces. If an unstoppable force enters an IO-universe, it can still be stopped by an Immovable object.

While not technically immovable rather than indestructible, Overduranium is usually the source of an IO-universe.

UF-universes
UF-universes can contain unstoppable forces, but can't contain immovable objects. If an immovable object enters a UF-universe, it can still be moved by the unstoppable force.

In UF-universes, some type of Deity is usually the source of an unstoppable force.

Contrary to popular belief, overduranium can exist in UF-universes as well, but it does not appear naturally. It can only be destroyed by the unstoppable force of the universe in question.

It is technically possible to reform Overduranium by bringing it to an UF-universe, but this is an incredibly difficult thing in itself, because IO-and UF-universes are located far away from eachothers in the Pigverse dimensional space. Even then, one would have to locate the unstoppable force of the UF-universe, and then find a way to utilize it.

Exceptions
Blank universes lack any type of matter or force, and remain outside of the IO-UF division until defined so.

The Alphabet Bird is capable of creating universes with both of these qualities. It avoids doing so in inhabited universes, because should an unstoppable universe ever meet an immovable object, the universe will cease to exist.