There are various levels in the LNH Imprint's cosmology.
The Looniverse contains the LNH's home planet, the Loonivearth, other planets in its solar system, like Net.mesis, and the denizens of worlds and galaxies beyond that, like the Dorfian Empire and Dvorakian Empire.
The Omnilooniverse itself is located on the newsgroup rec.arts.comics.creative, which also has a number of other Imprint Universes, like the ASH universe and the Patrol Universe. Sometimes stories feature travel to (or from) universes located in other Newsgroups; these universes are sometimes said to belong together in the Usenetverse.
The Astral Plane links many of these universes, and travel between them can be accomplished by the people who know the World's Address. It may or may not stretch beyond the Usenetverse, depending on who you ask.
The Real World
The Looniverse is a client world to the Real World. It draws strength and existence from it. Through dimensional sympathy, events in each affect the other.
Despite the many dimensional travelers in the LNH, along with characters like Fourth Wall Lass, who can traverse the Fourth Wall, no denizen of the Looniverse has ever visited the Real World; however, the Mechanical Author came closest before he was stopped. Theoretically, Fourth Wall Lass's powers after they were boosted with the Rung of Revamp could; however, she believes this could potentially destroy the universe. The Existents, however, say that any attempt to enter the Real World would simply put you in a version of Real World Minus One.
Real World Minus One is a version of the Real World that can be accessed by fictional characters, but even this is difficult.
Fifth Wall and Beyond
The Fifth Wall is the barrier that separates the Real World from the mysterious world that lies beyond even that.