Canon EOS Rebel T4i 18-135mm IS STM Lens Kit Instruction Manual page 231

Fully Automatic Shooting with Multiple External Speedlites
Multiple Speedlite slave units can be treated as one flash unit or
separated into slave groups whose flash ratio can be set.
The basic settings are shown below. By changing the [Firing group]
setting, you can shoot with various wireless flash setups with multiple
Speedlites.
[1All] Using multiple slave Speedlites as one flash unit
Custom Wireless Flash Shooting
Basic settings:
Flash mode
E-TTL II meter. : Evaluative
Wireless func. :0
Channel
This is convenient when you need a
large flash output. All the slave units will
fire at the same output and be controlled
to obtain a standard exposure.
No matter what the slave ID is (A, B, or
C), all the slave units will fire as one
group.
Set [Firing group] to [1All], then
take the picture.
: E-TTL II
: (Same as slave units)
231