Canon EOS-1D X Mark III Advanced User's Manual page 812

Note
For details on clearing all Custom Function settings, see Clear all Custom Func.
(C.Fn).
For details on clearing Custom Controls settings, see Custom Controls.
Still photo Custom shooting modes are reset when [Custom shooting mode (C1-
C3)] in [Reset individual settings] is selected in still photo shooting mode, and
movie Custom shooting modes are reset when this option is selected in movie
recording mode.
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