How are power levels automatically determined when using multiple speedlights with CLS?

Question

If I take a picture with an on-camera flash, or a single speedlight, my understanding is that the camera/flash pre-fire and then automatically select the approrpriate power level to properly expose the shot.

When you use two (or more) speedlights in a system (like CLS for instance), how do they automatically determine how much power each should use?

I can imagine plenty of cases where you would want one to be stronger than the other, would you have to set the power levels manually then?

I don't have any experience (yet) with off-camera flash, so I apologize if this is a stupid question, or if I have misunderstood how something like CLS is supposed to work.

Answer

In CLS, you organize your flashes into up to three groups (A, B and C; this is not the same thing as control channel). Each group is treated like a single flash. For groups set up to operate in TTL mode, during preflash, each group is metered separately (by camera, or by master flash in non-TTL auto mode), required power level is determined and during exposure each group is fired separately. You can mix up groups firing in TTL and groups firing in manual power.

On master flash, you can choose manual power setting or compensation level for each group and compensation level for the whole setup.

Check more discussion of this question.