Canon EOS-1 DX Mark III Advanced User's Manual page 755

5.
Select a registered file name.
Caution
An underscore ("_") cannot be used as the first character.
Note
User setting 2
Shooting after you have selected "*** + image size" (as registered to User setting 2)
will add a character representing the current image size to your file name as the
fourth character. Meanings of added characters are as follows.
"***L": ,
"***C":
"***M":
"***N":
"***S":
The automatically added fourth character enables you to determine image sizes
without opening the files after transferring them to a computer. Based on file
extensions, you can also distinguish RAW, JPEG, and HEIF images.
Movies recorded with User setting 2 are named with the fourth character as an
underscore.
Select [File name], then choose a registered file name.
, or
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