The Box is.
Generally speaking, the Box is the largest (im)possible, (un)imaginable, (non)theoretical, or use(ful/less) set. If you can think of something, it's inside the Box. If you can't think of something, it's inside the Box. Anything that is even declared without thought, subconsciously, is in the Box. The Box is in the Box. Logic, and all permutations of logic, are inside the Box as well, which simultaneously allows it to contain itself as well as deny characters or -verses any sort of excuse by saying "it doesn't conform to the Box's rules". They aren't the Box's rules, they just happen to be in the Box too.
It is impossible to define anything outside the Box, because by mentioning a fact about something, you have placed it in the Box. Lord Josh B. is in the Box, as is every -verse that has been written and could or could not be written, and everything else, even if it's not a "thing" at all.
Any excuse or explanation for how something is able to overcome or escape the Box is invalid, as by mentioning this ability or even the entity's existence at all you have placed it inside the Box. If you try to make multiple "The Boxes" they are all placed inside The Box.
Interestingly, the Box's name means that it is within the UNNAMEDverse. However, the UNNAMEDverse is inside the Box so it doesn't matter.
The Box is technically a concept of existence and non-existence.
Interestingly, if you went back in time you'd be in a different version of the box, but there's an infinite number of infinitely-nested Boxes. They are all self-containing.