Regarding the Separation of Universes
It should be recognized that travel between universes is not feasible unless through the use of a portal or similar unusual mechanisms. This is because it should be impossible for two distinct universes to be connected by a path that only goes through regular 3 dimensional space. Universes must be separated by something other than 3 dimensional distance or physical barriers, otherwise they would be considered to both be part of one large universe for our tiering purposes.
The most typical example is the presence of a higher dimensional space serving as a separator. In other words, two separate universes coexist in a four dimensional or even higher dimensional space, occupying different position along some additional dimensional axis. Therefore, in order to travel between such universes, the movement must be through the higher dimensional space between them. To summarize, two realms are separated through a higher dimension space if
- A larger space encompasses all the universes or space-times
- This space is of a higher dimensional nature.
While a common higher dimensional space is the most frequent way of separation, many other ways exist and are also valid.
In fiction there are cases where a wall or some non-physical analog separates two realms. However, while such a barrier might serve to separate the realms within the cosmology of said fiction, note that the spaces are not necessarily separate universes by our standards. That is because, while the wall may make it difficult in practice, one could in theory still move from one realm to the other with just regular three dimensional movement. That means that the realms are still part of one common three dimensional space, which by our standards constitutes only one universe.
Note that this criteria of separation is only an argument against two realms being separate universes, if a feat of travel between the realms by regular 3D means occurs or it is otherwise known for certain that they are not separated in the above sense. If the realms fulfill the requirements laid out in the prior section and no such travel occurs, then the realms can be considered proper separate universes even if the means by which they are separated is not explored.