Minecraft wiki entry for Allow_and_Deny, associated text: When placed, allow blocks let users without world builder status place or destroy blocks in the area above them, while deny blocks prohibit users from placing or destroying blocks in the area above them. This area covers an infinite distance above the block, but does not cover the area below the block. If two allow or deny blocks are placed on top of one another, the block that is highest takes precedence. The deny block does not prevent the placing of blocks with commands.