Canon EOS Rebel T4i 18-135mm IS STM Lens Kit Instruction Manual page 235

Setting the Flash Output Manually for Wireless Flash
When [Flash mode] is set to [Manual flash], flash exposure
compensation can be set. The flash output settings ([1 flash output],
[Group A output], etc.) that can be set will differ depending on the
[Wireless func.] setting (see below).
[Wireless func.0]
[Firing group: 1All]: The manual
flash output setting will be 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.
Other Settings
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