The Background Color Channel includes just the background as set in the scene, with the rest of the image rendered as black.
Overview
The Background Color Channel stores an image of the texture used in the background behind geometry in a scene as seen from the camera. Geometry that blocks the background image renders as black.
This render element interprets a background texture, such as the texture set in the Environment Overrides, as a background.
Other types of manually created backgrounds, such as a plane, mapped with a texture set outside a window, will not interpreted to be a "background" for the purposes of this render channel.
UI Path
||Node Editor|| > Add > Render Channels > Color Channel > Type > Background
Common Uses
The Background Render Channel is primarily used for layering the Beauty element (or combined render channel) on top of the background image. The Alpha channel can be used to create a matte for such a composite. In the following example the atmosphere and depth of field have been turned off for the sake of clarity.
Beauty Render Element without Background Render Element
Background Render Element
Beauty element with Background Render Element
Beauty element with Background darkened and colors tinted slightly in composite
Notes
When the background is behind a transparent or semi-transparent material such as glass or clear plastic, make sure the Alpha channel is enabled for the material for all render elements; otherwise, the background will not be visible in the render element. For example, when using VRayMtl for a glass material, set the Affect channels parameter for both Reflection and Refraction to All channels to cause the background to appear in the Background channel behind the glass.