In Nikon CLS, remote flashes are assigned to groups (typically A, B, and C). The system meters each group separately during the preflash, then the commander tells each group how much power to emit for the actual exposure. In effect, each group is treated like its own flash.

You do not have to switch everything to manual just because you’re using multiple lights. CLS can run groups in TTL and still let you bias them relative to each other using group compensation or ratio controls. For example, you can make one group stronger as a key light and set another group lower as fill.

You can also mix modes: some groups can be TTL while others are set manually.

A common setup is to place the key in one group and the fill in another so they can be metered independently. If both groups are left equal, the light can look flat, so many photographers reduce the fill group by about 1 EV or more for better modeling.