ahhh ok, I didn't understand, I thought it was something you have to do to set transparency.
so, if I've correctly understood, you set a trans- texture that will be used for alpha info, the darker the more transparent, but the transparency level will depend by bright level of the poly, caused by texture+ingame illumination.
last question: it isn't like a glowmap, you don't have a second texture that will be rendered in the high opacity areas, so what will happen when opacity is high? how will be rendered the poly? it will be like if only the bright (not considering the ingame light) parts of the textures are applyed?