When using multiple speedlites together you need to be able to control the power settings for each speedlite separately. Maybe one of the speedlites will be used as a main light, the second speedlite as a fill light to “fill in” or soften the shadows created by the main light and a third speedlite could be used as a hair light or a background light.
If you are using a very simple budget lighting set up you may be using manual speedlites such as the AmazonBasics Flash or the Yongnuo YN560 III or IV speedlites. To fire the speedlites off camera you can use a set of transceivers such as the Yongnuo RF603 – one transceiver fitted to the camera hotshot and another tranceiver attached to each of the off-camera slaves. The wireless transmitter only has the ability to fire the off-camera speedlites. You can’t change the power output of each speedlite. To do this you have to physically walk over to the speedlites and dial in the settings that you want. It’s easy to enter different settings manually for each individual speedlite. It’s simple but a bit slow.
With more fully-featured wireless systems (and more expensive) you often have the ability to control the settings of each slave speedlite from the master speedlite or controller unit. You can set the slave to either manual or ETTL auto metering and increase or decrease the power output. Some masters will even allow you to set the zoom focal length or allow you to use high speed sync flash.
In order to set the slave speedlite individually they need to be divided into groups.
Three groups will normally be sufficient for most situations:
Group A – Main light
Group B – Fill light
Group C – Hair light, rim light or background light.
By convention Groups A & B are best used for lights that will be illuminating the front of the subject while Group C is best used for lights illuminating the back of a subject (i.e firing towards the camera) or for lighting up the background. This is because when using auto ETTL II metering the groups A&B can be set as ratios and group C is set independently and basically ignored by the metering system.