Does that help a little?Note:
No mask or alpha channel is produced for the Specular Pass, so we recommend additively compositing a specular pass.
Custom shadow and reflection passes
You can also create custom shadow and reflection passes with the useBackground material which catches shadow and/or reflections.
When the shadow pass is rendered, a black image is created with mask channel that contains the shadow information. A compositor can work with this channel to blur, lighten, darken, etc., the look of the shadows.
When the reflection pass is rendered, an RGB image is created with a white mask in the mask channel.
To use the Use Background material to catch shadows and reflections, see the Lights and Cameras guide.