Plain: flat land, typically a field; also: the Great Plains
Plane: flat surface, typically a level board or table; also: "the cliff stood next to a plane of land."
Can you see my initial confusion? In essence, Plain is specific to land formations while Plane is general to all. So are they truly interchangeable or not? Further confusion. Obviously, when speaking it doesn't matter which one you're using because the meaning is implicit with the subject matter. My issue is substantial because I write science-fiction and fantasy. Let me further illuminate my dilemma:
Plain: Specific to land
Plane: a level of existence, a certain realm, a two dimensional coordinate system.
Plainswalker: derived to mean a Native American/Indian who walks across the Great Plains on a spiritual quest/journey of enlightenment and/or to commune with other planes.
Planeswalker: An inter-dimensional being who transcends other levels of existence or realms of existence.
A Planeswalker can walk from one plane to another; or from one plain to another. Visa-verca: A Plainswalker walks across the plains to connect with other planes; or transcends (mentally or spiritually) from this plane to communicate with other plains (speaking in terms of land).
Seeing this difference spelled out so definitively before me makes it seem entirely too simple, but my mind spins too quickly when writing or creating to care about the seemingly subtle difference. If I see (in my mind) a character sitting on a level surface (land or board) and he meditates to dissolve his energy/mind into an energy signal to translocate himself to a different place in order to communicate with someone standing, sitting, or working on a separate level surface (again, land or board), I don't decipher between plain and plane; though I'm sure there is a "more correct" use of the terms. This is something I am paying more attention to because I don't want you, my reader and critic, to be distracted by my diction. I just want you to see and feel what I'm writing about!
So here I stand, on a plane of concrete looking out over the small plain bordered by homes, wondering if i understand on which plane my mind plays as I wander to the final plane of existence. Will it be plains before my third eye, or mountains to traverse to freedom?