Paolo explains it nicely as usual! 

Let me just add from my own experience with Reality 3:

You can use all your skin textures you use in Poser. Reality will handle them beautifully. In some cases, as with M3, it fails to identify the skin and automatically set the material to "Skin", but that is easily fixed manually in the Reality material editor. For e.g. V4, the skin is handled perfectly without any manual intervention.

Reality will also apply Sub-Surface Scattering to the LuxRender "Skin" material, so no need to use advanced skin shaders in Poser for SSS. Just use a simple texture map for the skin and Reality will do the rest, SSS will be added automatically