(last updated: 12/28/17)
The Boundary is a realm of energy that possesses incredible information and is the blueprint for material reality. Within it, time and probability operates strangely, as all possibilities and permutations of the world exist, manifesting and disappearing in seeming heartbeats. Earthrealm is the only gateway to The Boundary. No other realm of existence is adjacent to it. The unfathomable potential of Boundary access is what makes Earth itself such a prize for otherworldly threats such as Outworld. It has always been there, unknown to nearly all existing characters, though a few have unwittingly utilized energy from it.
The Backyard as referred to in Guilty Gear lore is considered synonymous with The Boundary and the terms should be considered interchangeable. Just like The Backyard in Guilty Gear, The Boundary has tendrils that extend out to touch every point in time and is the medium whereby time travel, rare as it is in MotM theme, actually functions.
In most cases, even getting too close to The Boundary will be severely damaging on the bodies and minds of characters. Falling into The Boundary is akin to having all sense of self erased as time stretches to near infinite and even brief contact with its perimeter can lead to severe memory loss as they exist in countless dead realities at once, destroyed psychologically long before the soul succumbs. Those who survive this process often are turned into formless beasts lacking a blueprint, and will vanish from reality without outside interference. There are very few individuals who can venture into The Boundary without being thusly affected.
Gears from Guilty Gear and Murakumo devices from BlazBlue are conceptually similar as they are both created by a melding of Boundary Energy, science, and chi technology. Common Gears cannot last long in The Boundary, but powerful Gears such as Ramlethal and Justice can safely spend some time there with no adverse side effects. Murakumo units, due to the compressed souls inherent to their to their forging process, can exist within the Boundary indefinitely.
Seithr is chi that is heavily laced with Boundary energy. In dense concentrations seithr destroys the soul of most individuals over time, but when properly utilized, it facilitates powerful magic rituals or legendary seals. In MotM, seithr seeps into the world from major leyline junctions but does not drift far enough to be a danger to the rest of the world. It exists unobtrusively across the world in mild levels sufficient for minor use.