Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 210)

Clearing Flash Function Settings/Clearing All Speedlite
Custom Functions
1.
Select [Clear settings].
2.
Select the settings to be cleared.
Select [ Clear flash settings ] or [ Clear all Speedlite C.Fn's ].
Select [ OK ] on the confirmation screen to clear all flash settings or
Custom Function settings.
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