With an anisotropic dielectric, light is reflected and transmitted with a directional bias. The rotation parameter adds a directional bias to rays in ( 0-180 degrees useful range ) calculated in UV space.

When anisotropy is 0% the surface is isotropic ( with no directional bias ).

A brushed metal look is achieved by connecting a pattern map to the rotation parameter

[ QUESTION] Not sure how biasing occurs with transmitted light, reflectance seems to use UV space, but what space is transmission using?