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.
Turning on the multi-light value will render all layers with each of the lights used (It can also be Environment).
Multi-Light list supports interactive update in case you added, removed or renamed some light sources.