I think "IS" means either Intelligent or Interactive Surface
In addition to having more scale controls and material tabs for each face, collision works differently.
Non-IS shapes (Capsule, Cube and Sphere) can still collide with sim hair and sim clothing even when collision is turned off, where IS shapes don't.
I couldn't find out why this is or if there are any other differences.
Sorry to resurrect an old thread but just in case anyone else is looking for more info on this, like I was.