This spin control varies between 0 and 1 and defines the strength of the specular highlight region. A setting of 0 is a perfectly dull material and a setting of 1 is a very shiny material. This setting is used by the 3D viewer and the exporters as scale for a factor that is used as a power in the lighting equations. The factor can be varied for each type of exporter with a setting in the ini file (see Application Data and File Locations for information on where the ini file is stored on disk).
The shininess of a material describes how smooth the surface is. We associate small strong highlight area with very shiny materials and large weak highlight areas (or no highlights at all) with dull materials.