Canon EOS Rebel T3i 18-55mm IS II Lens Kit - Manual (Page 198)

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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 will differ depending on the [Wireless func.] and
[Firing group] settings.
[Flash exp. comp]
The flash exposure compensation is
applied to the built-in flash and all
external Speedlites.
[2 exp. comp.]
The flash exposure compensation is
applied to the built-in flash.
[1 exp. comp.]
The flash exposure compensation is
applied to all external Speedlites.
[A,B exp. comp.]
The flash exposure compensation is
applied to both groups A and B.
If [Flash mode] is set to [E-TTL II], you can press the button to
perform FE lock.
Other Settings
Flash Exposure Compensation
FE Lock
COP
Y