The second set of surfaces is “subtracted” from the first.

The portion of the first set of surfaces that is inside the second set of surfaces is trimmed away while the rest of the first set of surfaces is preserved; the portion of the second set of surfaces that is inside the first surface is preserved while the rest of the second set of surfaces is trimmed away.

If a surface is an open surface rather than a closed solid, the direction of the surface indicates the outside of the surface.