The Dink Network

I was messing around in the Dink Editor changing the hardness of a tile. I know that Z is the normal hardness (you can't walk over or 'fly' over), A is the hardness you use so that things that can fly may pass over, and X removes the hardness set, but what about S? While your editing the single tile, it shows up orange, but when your in hardness mode, it shows up purple. What is this hardness and what does it do?

Z - Normal hardness (no walk or fly over)
A - Fly hardness (fly over only)
S - ???