The Box contains everything that exists, as well as everything that doesn't exist.
If you can think of any concept, its going to be in the Box.
The Box also contains the Box itself, meaning it has an endless loop of self-containment. This means when you escape the Box, there will be another Box directly after it.
This means it is nearly mpossible to escape the Box, since no matter how far you go out, there will always be another Box.
The Box Axiom
Even though there is an infinite number of boxes, we can accept, as an axiom, that there still is a "set of all Boxes", just like there is a "set of all natural numbers".
This means, whether it is physical, metaphysical, pataphysical or nonphysical reality, a "thing" can technically still escape the Box due to this axiom.
Such an object is called a metaboxial structure. There are many of them, most of them have since been protected by smaller metaboxial structures.
The Box contains every concept possible, impossible, indescribable or nonexistent, so even metaboxial structures are kind of contained by the box, even if they aren't in their physical form.
These concepts allow the Box to still be a container of everything. The axioms don't apply, since concepts are not physical and therefore don't escape the Box's nonphysical barriers. It doesn't necessarily contain the clones of concept outside the box, but it contains at least one copy of all concepts imaginable and unimaginable.