Canon EOS REBEL T6i Instruction Manual page 190

Custom Wireless Flash ShootingK
Setting the Flash Output Manually for Wireless Flash Shooting
When [Flash mode] is set to [Manual flash], flash exposure can be set
manually. The flash output settings that can be set ([1 flash output],
[Group A output], etc.) vary depending on the [Wireless func.] setting
(see below).
190
[Wireless func.0]
[Firing group: 1All]:

The manual flash output setting is
applied to all the external Speedlites.
[Firing group: 1 (A:B)]:

You can set the flash output
separately for slave groups A and B.
[Wireless func.0+3]
[Firing group: 1All and 2]:

The flash output can be set
separately for the external
Speedlite(s) and built-in flash.
[Firing group: 1 (A:B)2]:

You can set the flash output
separately for slave groups A and B.
You can also set the flash output for
the built-in flash.