Canon EOS REBEL T8i Advanced User's Manual page 252

Wireless Flash Photography With Manually Set Flash
Output
Available when [Flash mode] is set to [Manual flash]. Available flash output options (such
as [ Flash output] and [Group A output]) vary depending on [Wireless func.] setting
details (see below).
[Wireless func.:
[Firing group:
The same manual flash output is set for all external flash units.
[Firing group:
Receivers can be assigned to flash group A or B, and you can set the flash output of
each.
[Wireless func.:
[Firing group:
Enables you to set the flash output separately for external flash units and the built-in
flash.
[Firing group:
Receivers can be assigned to flash group A or B, and you can set the flash output of
each. You can also set the flash output for the built-in flash.
]
All]
(A:B)]
+ ]
All and ]
(A:B) ]
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