To understand more about classification, read a volume rendering text. In my opinion, positioning the light in imagespace is more intuitive. The light is always white and always at infinity. It will stay in position as the volume is rotated under it. Add cutout centered at creates cutouts within the rendering.
Cutouts look as if part of the volume has been cut away with a sharp knife.
This means that you only need to check this checkbox and take a point within the volume that will then be the deepest corner of the cutout. The x, y, z coordinates are set in voxels. Render backfaces renders the backfaces of surfaces. This option may come in handy if the volume is very aliased, and you only see dark gray surfaces. If this gradient is negative, only the diffuse shading is added.