Reflectance channel
The Reflectance channel outputs the plain color of the Reflectance 0º and Reflectance 90º parameters of the objects in the scene. It shows the transition of reflectance 0º in camera facing angles to reflectance 90º in glancing angles and combines the colors of the different layers and BSDFs that may be present in the materials. It won't show the color due to Transmittance or Subsurface Scattering.
This channel is useful for compositing purposes.
The Reflectance channel (right) stores only the reflectance information of the material, excluding the shading provided by the illumination
Render on the left and Reflectance channel on the right - notice there aren't any shadows or highlights