(This is for the fanfiction "Crossover Together". Info may change from canon information)
The Surface World, also known as the Human World, or Overworld by Underworld beings, is a realm of reality that exists in between the Overworld and Underworld, acting as the common barrier, and main passing point between the realms. It is where Humans commonly reside.
The Surface World is highly diverse in climate and landscape, varying drastically depending on where one does go. Humans singularly rule this realm, and spread all across the realm.
As the Surface World is between the other two common realms, the Surface World and its human inhabitants are the main field for both Angels and Demons to meet, and to interact with humans. The Surface World is where, for the longest time, that Angel and Demon alike had to keep from learning of their existence, in accordance to the ancient rules established.