Unreal Development Kit Beginner’s Guide

Subtractive

So say you wanted to create a room. You could place six additive cubes making up the walls, floor, and ceiling, but there's a better way to do it. In addition to additive, unreal has subtractive, and it does just what you'd think—it carves a hole in additive. Select the builder brush, move it so that it's partially intersecting with your additive brush, and click CSG : Subtract.

If you move the builder brush out of the way, you'll see that there's now a chunk taken out of your cube, and there's a yellow subtractive cube in the 2D view.

Try moving the subtractive brush around. You will again need to rebuild geometry in order for the changes ...