Keep asking questions ;)

I seem to remember some squabbling when V4 was first released. Many textures simply have the eyebrows painted on to the skin_face material. 

On my V4.2 the eyebrow part is not visible and does not show up in raytracing.

It seems to be available to texture creators where changeable hair colors are desirable, or maybe different eyebrow styles.

 

its extra geometry with a matching material.