"2nd Skins" are the Geometrical Export of a Program:

 
Basically you feed the program whatever figure you want, and the program will export another Figure; that is exactly the same shape as the imported figure(down to the material zones,) but that is a "Skis" width larger.  Then you Conform the new figure to the old one.  You give the new figure a clothing material, and then a Transparency Map.  Wherever the Transparency Map is opaque you will see the clothing material/figure;  and where the map is transparent you will see your original figure's skin.

The trick is that you need  Transparency Maps, which are'nt easy to create.  All a Transparency Map really is is a Texture/UV Map.  But for where you've come by these, with Poser 5+, you can layer materials; over your original figure, whereby by allowing for more than one Material per Material Zone.

Creating with Poser, but without the 2nd skin application; makes for a smaller scene, and is slighter on your wallet.