Light Groups are a way to ascertain and modify how certain lights contribute to a scene, In a scene with multiple lights, you can set as many groups as you like each containing one or more lights. Lights can belong to multiple light groups and each group outputs its own buffers. Those buffers are:
Any light in a group will have their contribution removed from the main buffers with the same names, and those lights not in a group will continue to contribute to the main buffers.
Creating a Light Group
Groups are created in the Render Properties Buffers panel in the Edit.. dropdown.
Once you have made a group you can rename it in the list and choose the options for the buffers it outputs.
In this shot, we are in Final Render and can see the four spotlights projecting the LightWave logo onto the floor.
We have added the white and green lights to the Separation group and when we choose the Separation > Diffuse_Direct buffer we see only those lights:
If we look at the normal Diffuse_Direct buffer we see the contribution of any lights that are not in a group:
This allows you to control the contribution of your lights after you render, as shown in the example below.