In most situations, the zones for the contacted surfaces should be the same optical property so the rays interact with the surface the same way in both directions. However, there are certain scenarios where it is advantageous to be able to define different optical properties depending on the ray direction.
One case is when you want to model the appearance of a poster in LightTools. In this example, you want to overlay a spatial pattern (i.e., a picture) and some other reflective scattering behavior. To do this in LightTools requires a minimum of two surfaces. One surface has a zone with a spatial apodization pattern. The second surface is a reflective scatterer. Rays will pass through the spatially filtered surface twice, which doubles the filter’s effect.
By using two objects with air as the material, you can define an optically contacted interface where incoming rays see the spatial filter, reflect off the scattering surface, and return through the filter surface a second time unaffected.