Hi,
If I understand correctly AT-ST can move and overhang onto terrain lower than itself if it's stable.
Imagine situation like this:
Black rectangle is terrain, orange AT-ST, in that case:
1 Can units shoot thru space marked by red dots?
2 Can units move into space marked by red dots if terrain is higher than unit siluet?
3 Can vehicle move onto terrain marked as Key position and block access of any other unit?