Commander Mode - Groups A and B

I haven't touched my D90 for a while and everything I leaned is now almost forgotten. Anyway, I have an SB 600 flash and want to use the CLS system. What I am confused about is the reference to Groups A and B whilst in Commander Mode and whether or not they should be in TTL mode or not. What are Groups A and B? I am an old fogie so please be kind.

Thanks

Brian

The groups come into play when you want to use more than one flash. They give the option of reducing power on one group independent of the other. When using one speedlight, just make sure that it is set to the same channel and group as the commander and turn the unused group off. You can use TTL or manual. If you don't want any part of the scene lit by the pop-up flash, set it to commander mode only and preferably use a SG-31R shield mounted in the hot-shoe. The pop-up gives off an afterglow that can be visible in some situations.

Thanks very much for that. That explains things that even a simpleton like me can understand.

Brian

Thanks very much for that. That explains things that even a simpleton like me can understand.
 
Brian