Canon Speedlite 600EX II-RT Manual (en)

To perform radio transmission wireless shooting, set the master unit 
and slave unit with the following procedure.
Set to <'M>.
Press the <I> button.
Turn <9> to select <'
then press <8>.
Set to <'x>.
Operate and set the flash you want to 
set as the slave unit.
Select <'
> in the same 
way as for the master unit setting.
To avoid interference with wireless flash systems using radio 
transmission used by other photographers or with other devices using 
radio waves (wireless), you can change the transmission channel and 
wireless radio ID. Set the same channel and ID for both the master 
unit and slave unit.
Wireless Settings
Master Unit Setting
Slave Unit Setting
Transmission Channel/Wireless Radio ID
To perform normal flash photography, select <F> to clear the 
wireless (master/slave) settings.
When establishing multiple radio transmission wireless flash systems in a 
place, interference between flash systems may occur even if the flashes are 
set to different channels. Set different wireless radio transmission IDs for 
each channel (p.64).