Nodes are the essential building blocks in the Advanced interface to the Poser Material Room. They are the graphical representation of mathematical function calls, that is: calculation procedures which turn parameters (inputs) to a result (output).
Like all special nodes, the FastScatter node also provides diffuse, specular and eventually more as an all-in-one offer. Just black-out (or nullify) the regular Diffuse, Specular and other properties of the PoserSurface, and plug the node into Alternate Diffuse.
FastScatter is an early, simple – and according to the manual outdated – way of representing SubSurface Scattering. It does not use the subsurface scattering as referred to in the Render Options.
Besides a single Color entry, the choice is for None (demonstrated in the right pawn base in the image below), Light (pawn column) or Strong (pawn head) material. The left pawn in the render shows regular diffuse and specular, for comparison.
This article presents a comparison of the various skin and scattering nodes.