Canon EOS REBEL T6i Instruction Manual page 189

Flash Exposure Compensation
When [Flash mode] is set to [E-TTL II], flash exposure compensation
can be set. The flash exposure compensation settings (see below)
which can be set vary depending on the [Wireless func.] and [Firing
group] settings.
FE Lock
If [Flash mode] is set to [E-TTL II], you can press the <A> button to
perform FE lock.
Custom Wireless Flash ShootingK
[Flash exp. comp.]
The set amount of flash exposure

compensation will be applied to the
built-in flash and all the external
Speedlites.
[2 exp. comp.]
Flash exposure compensation is

applied only to the built-in flash.
[1 exp. comp.]
The set amount of flash exposure

compensation will be applied to all
the external Speedlites.
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