In the Unreal Engine, a Subtracted Brush creates
an area of void, which the player is then able to move in. As any
environment in the Unreal Engine starts off as solid space, at least
one subtracted brush MUST be included in any level
that is playable, as without it, there would be no physical space
for the player in the environment.This is a Solid Brush, and it makes BSP cuts to the level. A Subtracted Brush that is placed within a solid area will create a hollow area. However, if it is placed within another hollow area, it will not create a double subtracted area, but in fact has no further effect.
This brush type is used in conjunction with the Subtractive CSG Operation.
The Subtracted Brush will only change after a rebuild has occurred, as it relies on the CSG Operation to create the effect that it has, and CSG Operations only occur during the rebuild itself.
The Subtracted brush, like all other brushes (except the Active Brush - which is a special editor brush) takes its form from the Active Brush when a CSG Operation is performed.
To place a Subtracted Brush into your level, shape the Active Brush and place it where you want the Subtractive Brush to be, then click the Subtract button. (shown below).
This will create the Subtracted brush.