It's a *really* good idea NOT to use cgs subtract. There's a high probability that using it will result in any number of invalid artifacts being left behind that will screw up your level come compile-time. I made the mistake of using this "feature" when I first got 3DGS, and I built my entire level using it often, and then discovered what a mess it had made and I had to rebuild everything again.
I wish they'd just remove the feature altogether so others don't make the same mistake (especially if they haven't checked the forums here first)...