Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 755)

5.
Select a registered file name.
Select [ File name ], then choose 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 ”: , , or
“ ***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.
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