Canon EOS REBEL T7i Instruction Manual page 226

Custom Wireless Flash PhotographyN
Fully Automatic Shooting with the Built-in Flash and
Multiple External Speedlites
The built-in flash can also be added to the wireless flash photography
described on pages 224-225.
The basic settings are shown below. By changing the [Firing group]
setting, you can shoot with various wireless flash setups of multiple
Speedlites complemented with the built-in flash.
[1All and 2]
226
Basic settings:
1
Flash mode
E-TTL II meter. : Evaluative
Wireless func. : [0+3]
Channel
Select [Firing group].
2
Select the firing group, then set the

flash ratio, flash exposure
compensation, and other necessary
settings before shooting.
: E-TTL II
: (Same as slave units)
A
[1(A:B)2]
B