The Multi-Light section of the Image Layers group of settings.
The Multi-Light section lets you render separate lighting element contributions at once. Selecting one or more lighting elements in the UI will produce as many additional outputs for each shading component image layer in the Image Layer list. Each output will only contain the light emitted from the selected lighting element. This powerful feature lets you then composite lighting elements in comp or mix them using 3Delight Display.
An example of how this feature can be used is detailed in Multi-Light Rendering.
The elements of the multi-light list are presented as follows:
Environment
Selecting this will produce an image layer with the light contribution of the Environment, as set in the Scene Elements section; this item is insensitive when no environment is set in Scene Elements.
Lights
Lists the light sources that are not in any Groups.
By default, the Lights section only offers light sources that are not members of any Light Groups. This is the default behavior because attempting to composite lighting elements which have light sources repeated will yield incorrect results.
Multi-Light list supports interactive update in case you added, removed or renamed some light sources.