"Reality is that which, when you stop believing in it, doesn't go away."
Philip K. Dick
Position on the Omniumverse
It contains everything else
Both the Omniumverse and the Imaginarium are real in some sense of the word, therefore part of the Realium.
The proof of that is that we truly imagine things that are not real, therefore imagination is real. As the Imaginarium is just another name for imagination, this shows that it is also real, even if not in a concrete material way.
The same thing can be said of the Omniumverse. Its essence is that of whatever difference exists between what is actually real from what is imagined. It is an obvious truth that there is some difference between what is really real and what is fantasy therefore the Omniumverse as defined must also be real.
Reality is also in the Imaginarium
Anyone can theorize about the internal organization of the Realium but, until those theories can be verified or proved, they just describe one more Imaginata inside the Imaginarium.
Even if they turn out to be truth, they also already exist as Imaginatas as by the previous statement and knowledge also exists in the Imaginarium so they would continue to exist as Imaginatas anyway. This implies that there exists an infinite number of copies of the Realium in the Imaginarium. Or, more than just copies, that the Realium actually exists an infinite number of times inside the Imaginarium.
Both the Realium and most Imaginatas, but not the Imaginarium, have some type of internal organization. This internal organization is called a Cosmology, no matter how or what it is, which corresponds to their inner reality. The closer a specific Imaginata is to the Realium, the more based on it it is, the more similar its internal organization will be to that of the Realium.
If, instead of a single reality, there exist multiple parallel (or not) realities, they do not, each, correspond to one Realium out of many. Instead the Realium is the collection of all true realities, no matter how different or "far apart" they are from each other in whatever way.
Point of View Hypothesis
If one ignores the fact that one is real and the others are not, there is a similitude between the Realium and the Imaginatas. It is possible, even likely, that from their inside, many of the Imaginatas look to be the Realium instead and that the Realium is, for them, just another Imaginata.
So, what is the Realium and what is an Imaginata can just be a matter of a point of view. This is not a certainty but it is a possibility. This can make one wonder if there is actually any difference at all. Or is it just not another case of we putting ourselves at the center of reality or in an otherwise special position like putting Earth at the center of the Universe? Is our reality special and different because ours is the real Realium and others are just Imaginatas? Is "real Realium" just a pleonasm, exactly like it sounds or is Realium just another word for "my Imaginata"?
If it is the case that the Realium is just another Imaginata then the Omniumverse would just be the Imaginarium and nothing more, instead of being something larger that contains it, with the Great Barrier being just the border between each Imaginata and the rest of the Imaginarium. If that is the case there is no way to be sure of it while staying inside of either the Realium or any other Imaginata and it is not possible to know if there will be a way to be sure even if standing directly in the pure Imaginarium.
With The Box being synonymous to the Imaginarium in all likelihood, that would mean that it also turns out to be everything after all.
Comparisons with other structures
There have been some attempts at comparing the Realium with other structures that are also about defining or placing reality in relation to the rest of a cosmology. For that purpose, it can be said that any structure that corresponds to all reality and only to reality alone, that is real only and always, from any and all points of view, no matter what, is the Realium.
If there is no such structure that corresponds to reality from any and all points of view then, as explained above on the Point of View Hypothesis section, there would be no Realium. All possible realities would have a point of view from which they would be fictional instead. There would be no absolute reality, therefore, no Realium.
Creator God Hypothesis
If there exists a creator god of reality, one could identify its act of creating reality with the act of anyone else creating an Imaginata through the use of imagination. This would allow us to say that the reality itself, the Realium, or even the Omniumverse is just the creator god's Imaginata. Such creator god would be reality's Demiurge.
Realium in Practice
What this all means in practice is that the Realium, by definition, is off-limits to any and all fiction or fantasy.
Everyone is free to write, believe or imagine whatever they want about the real world, any story, any speculation, etc. But the thing is, that as soon as they do so, they are actually writing about a fictional version of reality, not about reality itself. They are writing about just another Imaginata, no matter how similar it is to the Realium, it no longer is the actual Realium. It no longer is the actual reality, it is a fictional or fictionalized reality.