What's the meaning of the secondary color?

What’s the meaning of the secondary color? I had used from the glColor() before, But in the OpenGL 2.0 specification, i read some texts about the secondary color and glSecondaryColor(). How can i use from the secondary color in my code? What does the function glSecondaryColor() do?

Secondary color is interpolated across polygons and can be consumed or produced by vertex shaders and consumed by fragment shaders.

You can also use it in some texture combiner extensions even without shaders.

It is a generalization of the fixed function pipeline capability to apply specular highlights after texture modulation (originally an HP extension). Just think of it as another color you can use in interesting ways with advanced texture features and multitexture, although I think it has no alpha and was explicitly excluded in vanilla ARB_combine.