Underwater Lights For Iray - What's The 'Base' Setting For?

Xenomorphine

The bonus of this being able to depict bloody underwater scenes, made this an instant purchase! However, as I'm looking down the options, each of the five varieties of lighting also has a 'base'. It's not a shader or HDR texture for the environmental dome, but another light.

Should this be applied before we use one of the other five varieties? Or should it just be treated as the darkest possible version of each?