Operations

Every brush, generator and composite has an operation that says what it does to the shapes above it in the hierarchy:

Operation	What it does
Additive	Adds its shape
Subtractive	Cuts its shape out
Intersecting	Keeps only what is inside its shape

Order matters: a shape only works on the shapes above it in the hierarchy (within its group), so a subtractive brush only cuts the brushes above it, and an additive brush below it fills the hole again. Nothing is ever destroyed: you can change an operation, move a brush or reorder the hierarchy at any time and the level is rebuilt.

To apply an operation to several brushes at once, group them in a composite: see Hierarchical CSG.