Okay, I think I have finally found out what the difference is. As described the spec file touches the glossiness. The env_mask seems to create a crystal or billard ball. This effect is good for eyes, I think, maybe also sweat. Both effects, specularity / glossiness and environment mask, are increased the brighter the map at the corresponding point is and decreased by a darker value.
Another question: I created a nrml map with an nvidia plugin, but things still looks flat. Are there some important properties of a nrml map that I should switch on or give a certain value? So far I set MinZ to around 10. This time I even set it to some higher value (20 or even 30, I don't know anymore... It's some time ago that I did it). Suggestions?