Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 753)

File Naming
Registering/Changing File Names
File names consist of four alphanumeric characters followed by a four-digit file number ( )
and file extension. You can change the first four alphanumeric characters, which by default
are unique for each camera and set when the camera is shipped.
User setting 1 enables you to register four characters of your choice. User setting 2 adds
three initial, registered characters of your choice to a fourth character representing the
image size that is added automatically after you shoot.
(Example)
Registering/Changing File Names
1.
Select [ : File name].
2.
Select [Change User setting*].
753