Canon EOS-1D X Mark III Advanced User's Manual page 676

4.
Select [OK].
The file name is determined automatically by the camera: WFTNPF,
followed by a number (01 to 40) and the extension NIF. To rename the
file as desired, press the <
The settings are saved to the card.
The settings file is saved to an area of the card shown when the card is
opened (in the root directory).
Caution
40 settings files can be saved on one card from the camera. To save more than 41,
use another card.
> button.
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