if I may try: when light hits a surface, it gets sort of captured and re-distributed again, but not in equal amounts in all directions. As a result, surfaces which face tha camera are brighter than surfaces which make a skew angle, and so, objects appear darker at their sides than in the middle. I toss "shading" as the word for it (to separate it from shadowing which is something different).

Regular Poser diffuse as well as specular implement the "Lambert method" for this angle-dependant intensity falloff. Which calculates pretty fast, is not that bad for the diffuse part of smooth surfaces, but is far from realistic for porous surfaces and for specular, turn everysurface into a hard plastic one.

So Poser offers alternatives, and has made some improved versions of those over the years. Clay is the alternative to diffuse, and Blinn is the alternative to specular. Diffuse does not have other main alternatives, specular has Glossy and Phong which are considered outdated, Anisotropic which is meant for surfaces "with a direction" like brushed metals or vinyl disks, and the recently added Ks_Microfacets. The latter is not available Poser 8/Pro 2010 or earlier so content vendors are not that willing to implement it, and there is not that much experience in its proper settings and behavior of parameters. So, when looking for a decent alternative to specular, Blinn is the well understood alternative of choice, especially for vendors.

my 2ct

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though