Concrete Back Wall

 is a wall block that can only be placed on the background layer and only supports itself. You cannot place wooden platforms, Iron Supports or any other object onto it directly, though it will support wires. By placing them, the player effectively prevents monsters from spawning where these back wall blocks are placed.

Properties
 is a useful base construction material because it prevents the spawning of monsters inside that tile. Its construction is possible in the Auto-Builder MK II.

The  does not support anything other than itself (and wires) and therefore isn't useful for structural support.