Planes
A Plane or Plane of Existence is a flat surface with infinite width and length. There are only two planes in the Kingdom Hearts universe. These planes have been dubbed with the names of the Realm of Light and the Realm of Darkness. The worlds that are visited in Kingdom Hearts belong to one of the two planes with the exception of Castle Oblivion, Twilight Town, Yen Sid's Tower and The World That Never Was which exist between these two planes in the Realm of In Between. The Realm of Light contains all the 'normal' worlds such as Destiny Islands, Deep Jungle, and Halloween Town. The Realm of Darkness is only known to contain Kingdom Hearts. It can be conjectured that the Realm of Nothingness takes place outside of the planes and not in the between area. These planes or realms should not be confused with worlds, however tarveling from plane to plane or to the places in between the planes can be done via Gummi Ship, Corridor of Darkness, and even stairs just like worlds. Traveling between planes can also be done via Doors, such as The Door of Light and The Door of Darkness. The planes as well as the areas outside are a part of the Sea of Outer Space.
Stairs
Stairs were only mentioned once by Tetsuya Nomura's in the Directer's Secret Report XIII. Tetsuya Nomura's stated that to fully imagine the Kingdom Hearts universe correctly "you can imagine 2 planes with stairs ranging between the worlds [on the planes]". Stairs being used as a means of inter-world travle has only been seen once during the opening scene of Kingdom Hearts II. Sora was seen climbing stairs in order to reach Castle Oblivion.