A scene that demonstrates how to use the imager_light_mixer can be found here.
The light mixer operates on light groups not individual lights, so first you need to create groups via the Light Manager.
Light groups displayed in light_mixer. Lights without a light group display as default.
Adds an existing light group as a layer in the light mixer.
Opens the Light Manager window where you can create, edit and delete light groups.
Mute an individual layer by Ctrl + click on the mute button (exclusive mute).
Solo an individual layer by Ctrl + click on the solo button (exclusive solo).
More information about light groups can be found here.
When enabled, creates a new additional AOV in which the mixed result will be written into, instead of overwriting the beauty.
Note that this feature is available only in multi-layer Arnold drivers (e.g. EXR) and when rendering to the Picture Viewer, but in single-layer Arnold drivers (e.g. jpeg, png, etc).
The name of the AOV in which the mixed result will be written.