I believe what they are referring to here is the ability to specify another material for the sides of terrain cliffs. A lot of terrain materials do this to aide in realism. Without this, a whole material slot is required and developers must manually paint the sides of mountains and cliffs. For games already using every material available, it is not possible for them to override another material for the sides of cliffs because they are already using that material elsewhere in their game.
This is probably solved down the road with adding new materials, but I don’t remember if this was something Roblox wanted to do.