Also known as the 4th wall, the Great Barrier is the barrier between reality and fiction. It is what makes those two things different and separate. Without it there would be no difference and anything anyone imagines would be as real as they are.
Its place in the cosmology
It exists not only around the Realium, separating it from the Imaginarium but also around each and every Imaginata, separating its reality from the surrounding Imaginarium as well.
From the point of view of any Imaginata, it is reality and the Realium is just another Imaginata inside the Imaginarium. There is no fundamental difference between the way the Realium is separated from the Imaginarium and the way any Imaginata is separated from the Imaginarium. That difference is only in the point of view. What reality one belongs to will always look like the Realium for oneself.
The only thing that varies is if people accept other Imaginatas as being other alternate realities that exist in the same way and are as real as their own, or if they view them only as fantasies in a sea of fantasy. This would support the view that the Imaginarium actually is the Omniumverse.
Imagination goes through it
As it occurs in our reality, it seems to be extremely difficult, or even impossible, to actually cross it in a physical way.
Even then it remains somewhat permeable to our thoughts and feelings. The act of imagining something is the act of crossing it to the other side with our minds. If it where not permeable to our thoughts we would not be able to imagine anything. We send our thoughts to the Imaginarium and they gain form in there. We mold them to our desires and fantasies and they continue to exist in there even when we take our minds and thoughts back to our reality.
Things can also cross through in the opposite direction
Our imaginations sometimes run out of our control, existing by themselves following their own logic or desires without being defined by us. This phenomena can be witnessed every time an author writing a story has a character that starts to act on their own accord, no longer in full control of the author who limits themselves to writing what they perceive as happening on that fantasy world that just escaped their full control.
The images of our fantasies also cross back into our minds for that is the only way that we would be able to remember them.
We are constantly affected by the fantasies existing out there on the other side of the Barrier. Our feelings and thoughts are affected, we feel real emotions because of what we have imagined, we have new ideas and inspirations, etc. Many scientific discoveries have been made through inspiration. What is that if not some image or piece of information crossing back through the Barrier into our own reality and directly into someone's brain?
On other Imaginatas the Barrier can be different in many ways, have different properties. It can even have different properties along different places inside the same Imaginata. The only commonality is that it separates its internal reality from the Imaginarium in some way.
Different Imaginatas probably have Barriers with very different properties. They can have any property with those listed here being only some of the most common or obvious examples.
It can be more or less permeable, to our minds, thoughts and fantasies, or even to matter itself.
An Imaginata with a non-permeable Barrier would be a place without imagination. People are not able to imagine things at all in such a place but it can be even worse than that. It can prevent people from conceptualizing, thinking about or even remembering things if they are not able to access what exists only in their minds in any way. It is hard to imagine existence and sapience without such abilities but maybe they can exist somehow. If they do, they are probably very rare.
The other extreme is when the Barrier allows anything to go through it. It becomes possible for matter, even people, to go through it in some way, leaving their reality and entering the Imaginarium directly, possibly reaching to other realities through it. It is no wander that the Imaginatas that allow this tend to be the ones where the view that theirs is only one of many possible real realities are the more common.
The Barrier can be permeable to one or more senses, allowing people to observe the chaos of the Imaginarium on the other side somehow.
If it exists as the edge of reality, one just needs to look far away but it can also be possible that it requires some special machines that allow its observation, like piercing through the dimensions, through the fabric of reality and outside of it by some television like screen or some telescope like thing. Instead it could require another special sense or maybe be observable only by some animals that would have that special sense or the normal senses acute enough for it.
It would probably be possible to see other Imaginatas at a distance, floating around on the Imaginarium, possibly even looking into them if their Barriers and the conditions on the Imaginarium allow for it and if one is able to have enough resolution to do so.
There is the possibility that our own Barrier has some degree of transparency to those who believe to have some form of sixth sense or mediunity and that the related types of so called paranormal phenomena are just the perception of the surrounding Imaginarium through it.
Its properties can change over time, possibly in relation to something inside or outside the reality it isolates.
A typical example is for the Barrier to become less permeable, more rigid and harder to cross as logic and science become more developed and common inside it. As people's ideas of their reality are less based on fantasy and they use their imaginations less and less to define it, the Barrier becomes harder and harder to cross, making the distinction between fantasy and reality even bigger and more evident.
As people understand the difference between fantasy and reality better they also define and enforce that difference more strongly. This is a type of subjective reality where reality becomes more and more objective as it is perceived to be so.
On other Imaginatas where people still define their own reality in a magical way, based on myths and legends and wonder, the Barrier remains much more flexible, enhancing that world view.
Not all Imaginatas have their Barrier being affected by the way people inside it view reality but that is probably the most common case. On the other cases the Barrier is the way it is no matter how people view it or reality.
Transforming whatever goes through it
When this is a property of the Barrier it can work when crossing it in both directions or just one. And it can be automatically reversed when crossing it back again or not.
The changes can be very drastic or minor, can be physical or mental, etc. A Barrier that changes the nature of everything that goes through it can allow for easy crossing but most people would not desire to do so unless they are able to devise a method of protection.
Such a property, if understood well enough, could be used as a means to liberate your trans-humanistic desires or it could be used as a form of punishment as could banishment from reality itself through the Barrier. The transformation can be in such a way as to make it very desirable. Maybe it gives some advantageous property. If that is the case, if it has no risks or unwanted consequences and if it is easy to do, probably entire populations would do it as much as possible. On the other hand, if going through it would cause death, unless one is wearing some kind of protection, it could be used for suicides or as a form of execution.
Another option is a Barrier that transforms everything that goes through it to enter the reality inside it in a way that makes it compatible with said reality. The reverse could be a Barrier that gives imagination like properties to everything that leaves the reality inside it, making those things susceptible to being changed or affected by the imaginations of those inside it.
The common case
As tends to happen, we usually consider ourselves to be the norm, and this is no exception so we would expect that the most common case is for the Imaginatas to have a Barrier similar to our own. Allowing them to imagine things out in the Imaginarium but, apparently, not much else.
In many cases, especially those where the Barrier allows for physical crossing in some way, it has a physical manifestation inside the reality it is isolating from the Imaginarium.
These manifestations can be anything, from subjective and metaphoric things to concrete physical ones. It can appear as a physical barrier of any type, like a huge chasm, an impenetrable darkness, a wave of gravity or maybe of negative gravity which repels everything, a mystical barrier of energy or even a brick wall that exists somewhere. It can even take the form of things like a black-hole or maybe all black-holes existing in the universe, a mythical fountain on a lost land, a revered river or dangerous sea. It can be the light of the stars or the perfume of the flowers. It can exist as a wall of thorns or a wall of fire. Maybe it is every fire everywhere. It can also be the actual space-time fabric of existence.
Some believe the Mists to also be a manifestation of the Barrier.
For smaller Imaginatas it sometimes manifests itself as the actual edge of the world. Either an obvious barrier or an actual edge that allows one to look into the Imaginarium over in the distance.
The Barrier and its manifestations are never limited by the laws of reality for it is not part of the reality inside it. It separates reality from fiction but it is part of neither, while, at the same time, being part of both so it is subject to the laws of none of them.
Crossing the Barrier
Crossing the Barrier itself can be more or less hard, requiring anything from just stepping on a certain direction to needing complex and powerful magical rituals or very advanced scientific understanding of the nature of reality allowing for the development of the needed technology to do so.
These technologies can be in the form of portals opening to other realities or to stabler close by places of the Imaginarium or in the form of ships that allow entering the Imaginarium in a "safer" way, providing a bubble of stability inside them for the protection of their crew. Other possibility is by using brain to brain interfaces or by using machines to mediate such links through brain-computer interfaces in order to allow one conscience to enter the imagination worlds that exist or have been created by another mind.
The more physical and obvious and concrete it is the more obvious the way to cross it is. One just needs to perform the action that would typically allow crossing of such a physical obstacle, possibly with a greater intensity but the analogy remains. Breaking through the great brick wall, using a boat to cross the ocean, etc. In other cases it is more complicated and it enters the realms of mysticism.
When it is possible to cross it in some way, some manifestations of the Barrier can cause unsuspecting persons caught by it to cross it accidentally, possibly dropping them on other parts of their reality, other realities or even directly in the Imaginarium. These events can be described as the poor victims having fallen through the cracks of reality.
Danger of No Return
Even when it is possible to physically cross the Barrier in one direction it doesn't necessarily mean that it is also possible to cross it back on the other direction. Sometimes the journey is a one way trip and there is usually no way of telling if that is the case beforehand. Even if just crossing it with one's mind it is perfectly possible to become trapped outside as a disembodied mind, leaving nothing but a comatose shell behind.
It is possible to imagine that death itself is one of such one way trips across a Barrier that does not allow for the return trip into our normal physical reality.
Magic and the Barrier
Most magic everywhere tends to use the power of the Imaginarium, consciously or unconsciously, directly or indirectly, with or without knowledge. A common thing to most magic is that it usually has some kind of what could be called a price or a consequence for those who use it. These consequences usually come from the need to go through the Barrier in order to use or collect such magic power. Even if one is not aware that that is what one is doing.
When fantasy leaks into reality
If the Barrier is so thin as to practically be non-existing, the Imaginatas inside it get flooded by the Imaginarium and that can be very dangerous, possibly fully destroying them but, at beast, leaving them in a state where many of the chaotic and undefined properties of the Imaginarium apply to them as well. Fantasy and reality become indistinguishable or almost so. Anything anyone imagines becomes as real as they are and that can be extremely dangerous if uncontrolled.
In some Imaginatas the Barrier is either flawed in some subtle ways having holes on its structure that allow unexpected passage through it or is permeable enough to the fantasies of imagination to be able to enter it without it being so easy to the point of being ridiculous, so it happens rarely enough that a distinction between reality and fiction is maintained.
A fictional creature or event can enter it once in a while or a place can be slowly affected by the thoughts and beliefs people have about it. Feelings, ideas, imaginations about an item or a location or even a time can slowly impregnate such things and change their real properties over time.
This usually results in different types of myths and legends. A hunted house, a character from a book roaming the world in disguise, ghosts and other stories. A book that leaves all its readers perturbed by it in an unexplained way, without a logical reason to do so. All types of things can be attributed to such fantasy leakage into reality.
In some Imaginatas, advanced technology and/or powerful magic can allow the Barrier itself to be affected in different ways.
It can be reinforced or weakened. Its properties can be changed in any way, possibly opening holes in it, making it easier to cross in specific places or ways for those who know how to use those passages. Making it denser and harder to cross or giving it some defense mechanisms, to act as a protection from some danger from the Imaginarium threatening their reality. Possibly making it scan everything that enters it, for virus, weapons, excessive power, even bad intentions, incompatible features, or other possible hazards. The risk of the Barrier allowing an Incompatible Being to enter through it is too high. Luckily such beings tend to be rare enough in the close neighborhood of any Imaginata.
Linking the Barrier of two different Imaginatas to facilitate travel, communication, commerce, etc between them. Possibly by opening related holes in both Barriers that tunnel through the Imaginarium directly to each other. Sometimes networks of such tunnels crossing each other can be found in the Imaginarium, linking many different Imaginatas. Sometimes the tunnels divided into several forming very complex networks. There are even way stations connecting directly to openings on such tunnels at some points.
Altering the Barrier of another Imaginata to allow easier entrance into it or maybe making it dangerous or even lethal to those inside it, turning it into a weapon or a tool of war and conquest. By completely blowing up or ripping apart a Barrier from the outside one makes the entire reality inside it open to the Imaginarium, free for the taking by anyone capable. Everything inside it waiting to be mined for resources.
Anything is possible for those with the ability and will to engineer entire realities to achieve their goals.
There is also the theory that whatever it is made of could be extracted and used to build other things. Its material is the concept of separation between reality and fantasy and what could be possible to do with that is anyone's guess. Maybe it can be possible to create new types of reality-fantasy dichotomies with some other properties.