Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 754)

3.
Enter letters and numbers of your choice.
Enter four characters for User setting 1 or three for User setting 2.
By selecting [ ], you can change the input mode.
To delete single characters, select [ ] or press the < > button.
Use the < > or < > dial to select a character, then press < > to
enter it.
4.
Exit the setting.
Press the < > button, then press [ OK ].
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