What we have for the flipside and its place in the fnaf world cosmology:
Fredbear:
"Something horrible must have happened on the flipside, because it’s causing this world to fracture. Objects like the tree behind me have broken." -The flipside is capable of causing reality to fracture.
To which 8-bit Fredbear answers: "Yes, the world beneath the surface world." -'World' here seems to mean like a universe, or at least in a universal scale
Continuing with Fredbear: "You need to find another sub-tunnel under the game, and it’s probably hidden inside another glitched object! Ahead are the Dusting Fields, you will probably find the next glitched object there." -The flipside's entrances are called sub-tunnels, and seem to be located inside the glitched objects. Also, this are 'sub-tunnels under the game', the game here being their reality. So, layers of reality.
"I do need to warn you of something, though. When you enter a glitched object into a sub-tunnel, it’s possible to find yet another glitched object inside. That will take you into a subtunnel beneath the subtunnel! There may even be more glitched objects there! Just be sure to never go more than three glitches down! I’m not sure if you would find your way back up!"- You can go up to three layers in safely, and 4 without return.