Opacity is also used for hair and clothing - for that, a map is created where the strands of hair or visible areas of clothing are white, the gaps are black and the inbetween sections are grey, and the map is then used for Opacity (if you were makuing your own you'd click the triangle next to Opacity and select Browse for image...): D|S would apply the set opacity to the white areas, make the black areas 0% opaque and set intermediate values for teh grey areas. You can see this in action if you load hair or a texture set for some clothing that cuts part of the original shape away and look at the Surfaces settings.