When you have multiple terrains listed under the same icon it will choose from the list.
" ": [ "t_grass", "t_grass", "t_grass", "t_grass", "t_dirt" ]
means it has a 4/5 chance of spawning t_grass and 1/5 to spawn t_dirt. As far as I know, that’s how it works, as an even distribution.
This is done mostly for grass and trees, so that it will result in a more randomized and therefore more natural-looking area.
I don’t think I know what you mean about stairs. If you put a down stairs on a floor and then put up stairs on the next lower floor, they will be connected, provided they have the same coordinates other than the Z coordinate.
IE: When you define your overmap special . . .let’s say we have cave_surface which is at 0, 0, 0. We put down stairs on the cave_surface map. Then we have cave_underground which is at 0, 0, -1. As long as cave_underground has an upstairs, and cave_surface has a downstairs, they should be connected.
I’m not sure how it works when there are multiple sets of stairs. I’ve never done more than the one set. I’ve also never used slopes, but it would make sense to me that they would function the same way.