The "normal" texture (bm file) is the bump map (pretty sure anyway). I can only assume the "da" file is the same as those used in the planet textures, which is for the dark side (opposite the light source/star).
The glossiness is just that. A lower number will make it less shiny. I changed all the asteroids to 1. They looked wet/glazed at their defaults, which looked funny.
The "cl" file is the actual texture, what you see in game.
