To See what this is do the following steps
1. Create a world in Minecraft Snapshot 17w50a, make sure cheats are enabled
2. make sure you use /gamemode creative @p to fly
3. use /locate Mansion
4. /teleport @p x y z (Make Sure You are flying because you need to be above the woodland mansion to see this).
5. You see there are no walls, floors, roofs, stairs, or ceilings