The Specular Gain controls allow the artist to break energy conservation for artistic purposes. If the values entered add to greater than 1.0 then the hair is not energy conserving. This is only important if you are unsatisfied with the look (it's too "hot") or are interested in starting with physically realistic shading.
The default gain for the below Specular parameters is 0.33, however, they can all be set to 1.0 and that is preferred as they are not additive. This default will change to 1.0 in a future releaseis now 1.0, previous versions may have been set to 0.33.
Primary Specular Gain
Gain for the R lobe of Marschner specular. This is like a clearcoat where the specular is fairly sharp and glossy and normally not colored. This specular is a direct reflection which is not tinted by any volume attenuation. Modifications of the R lobe help to make a hair look "wet" as well.