CSG Subtract: Works very much like a cookie cutter. It carves holes out of solid blocks using the selected object as the cutting shape. It removes the portions of other blocks intersecting the selected object. The texture of the cutter is assigned to the cut planes. This function is often used to 'chop off' pieces from blocks, or to break open the wall inbetween adjacent rooms to make a doorway. !! This function only works with convex blocks (not groups), and divides the block into a group of sub-blocks. The block texture must not be locked.