The per-vertex lighting model used in OpenGL assumes that the surface has microscopic facets that are uniformly distributed in any direction on the surface. That is to say, they assume isotropic lighting behavior.
Some surfaces have a directional grain, made from facets that are formed with a directional bias, like the grooves formed by sanding or machining. These surfaces demonstrate anisotropic lighting, which depends on the rotation of the surface around the normal to the surface. At normal distances, the viewer does not see the facets or grooves, but rather sees the overall lighting effect from such a surface. Some everyday surfaces that have anistropic lighting behavior are hair, satin Christmas tree ornaments, brushed alloy wheels, CDs, cymbals in a drum kit, and vinyl records.