Canon EOS R5 Mark II Advanced User's Manual page 211

Flash Custom Function Settings
For details on the Speedlite's Custom Functions, refer to the Instruction Manual of the
Speedlite.
1.
Select [Flash C.Fn settings].
2.
Set the desired functions.
Caution
EX series Speedlites always fire at full output when the [Flash metering mode]
Custom Function is set to [1:TTL] (autoflash).
The Speedlite's Personal Function (P.Fn) cannot be set or canceled from the
camera's [
Select the number.
Select an option.
: External Speedlite control] screen. Set it directly on the Speedlite.
211