well, basically...
in an "L" shape, in this formation
_|(interior)|_
the angle I am talking about is the spot where the vertical and horizontal line meet. This graphic was a bit difficult to find, because it was seperated from the other ceiling tiles, but it shouldn't be too hard to find, you just have to realize that it is how "|" connects to "_", meaning that it will be little more than a small piece, like "."
IDk... I'm confusing myself...
the interior angle is where, in a "+" formation, the graphic that will be at the center of the "+".
Ok, now that I confused you (and myself), on to the next topic!
the display image is what you will see when you select the graphic in the editor. It doesn't have any effect on how the game looks, it is just a way to tell you what the graphic does.
No use is a tile which, as far as I can tell, has no purpose whatsoever. It seems to be there to fill up space, and doesn't seem to have a purpose. Although i might be wrong, there isn't any reason to fill it in.
the pattern outline and pattern are exactly what you think they are.
EDIT: You will need a separate tileset for the corners, because RM can't do them with autotiles... That is, it doesn't support slanted walls. It only has 90 degree angles, and you will have to add the extra sides on a separate layer.