I noticed, after importing a few models with Raevn's wonderful script, that you guys are using entirely manifold geometry, except for joints. I'm curious as to why this decision was made, as I know that SupCom used non-manifold geometry, and for good reasons.
Actually, it's only on some models. So I guess it's just that everyone's rushed and some of the artists use subtractive modeling? Kind of odd, but it would happen under such a crunch.
There might be an optimisation pass on models at some point before release. Could just be artist preference.
No. Brushes are things like the mountain pieces, cracks, plateaus etc. Things that do CSG with the terrain. Features are things like trees/rocks and they don't need to be manifold. I think currently it might be spitting out errors if they aren't but it won't break anything.