The universe refers to all of space, time, and the matter within.
Its time is infinite and ever-present, its space is boundless and eternal, and its mass is limitless and everlasting. It is a collection of all that exists and all that is possible.
In theory, an infinitely large universe contains all possibilities.
But these are merely possibilities—much like how the physical laws and fundamental cognitive systems of Su Zhou’s hometown "Sealed Universe" and its neighbor "Entropy Universe" differ, their possibilities have collapsed into material entities.
Perhaps in the beginning, both the Sealed Universe and the Entropy Universe had the potential to become countless different forms of universes. However, once they were truly born, they set their own trajectory of potential advancement.
Nevertheless, these possibilities did not disappear after collapsing but became the foundation of parallel worlds, awaiting discovery by beings at the peak of Dao Integration and beyond.
These are all part of one universe.
The multiverse, on the other hand, encompasses the potential realities of infinite universes. It is a collection of all possibilities and impossibilities, parallel and unique spacetimes.
In other words, according to "strict human definitions," what is defined as the multiverse should naturally be "all-encompassing." Beyond this, there should not be any other spacetime. If there were, then the entity defined as the multiverse would not represent all possibilities and impossibilities. The existence of other spacetimes would mean it is not complete but rather a part of a larger "multiverse."