I'm pretty sure the manual or the tutorials go over flags. If I remember correctly, you can flag a certain side of a cube (Left, Right, Top, Bottom, etc.), and have the flag tell the engine not to texture that side when rendering. So, if you're making an indoor map, and you don't need to see the outside, you can flag the outside walls so they do not get textured (saves 50% of texturing) It saves video memory

-Kravenwolf