Canon Speedlite 600EX II-RT Manual (en)

[: Shooting in a Different Flash Mode for Each Group
Check the charge status and then 
When <G> is displayed, you 
can check the firing groups that are 
not fully charged by the indication on 
the screen on the left. For example, 
when firing group <m> is fully 
charged, the <m> indication in the 
screen on the left changes to <x>.
When all firing groups are fully 
charged, <G> and the charge 
status indication for the firing groups 
on the screen on the left disappear.
For other charge confirmations, see 
step 6 on page 69.
Each slave unit fires simultaneously 
in the respective flash mode set.
Firing group charge status
: Flash not ready
: Flash ready
When setting the flash mode to </>, make sure the slave units are 
the Speedlites supporting the Auto external flash metering. Firing in this 
flash mode is not possible if Auto external flash metering is not 
When the flash mode is set to <a> or </>, exposure is 
controlled as a single group to obtain the standard exposure for the main 
subject. If you shoot with multiple firing groups pointing toward the main 
subject, overexposure may result.
The order of firing among firing groups does not need to be consecutive; 
for example, A, C, E can be set.
When you do not want a certain group to fire, press function button 1 
<R> to set <v> when setting the flash mode in step 3.