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
1. A larger space encompasses all the universes or space-times
2. 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.