Unthinkable reality is the name given to the space that exists "outside" the Box. Given the nature of the Box as the absolute container of everything regardless of whether it exists or not, unthinkable reality is likely to be completely empty. This level of emptiness may allow for a better metaphor as being an infinitely spaceless shrink-wrap around the Box, unable to support any content of any kind whatsoever.
Whether unthinkable reality contains anything at all, beyond the Box, is debatable and unlikely, since the Box contains literally everything definable, undefinable, thinkable, and unthinkable an infinite number of times.
- The Box (all Boxes contained by unthinkable reality would in fact be the same single box)