Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 205)

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3 Changing the File Name
The first character cannot be an underscore “_”.
User setting2
When you select the “*** + image size” registered with User setting2 and
take pictures, the image size character will be automatically appended as
the file name’s fourth character from the left. The meaning of the image-
recording quality characters is as follows:
“***
L ” = 3 , 1 “*** M ” = K , 41
“*** N ” = 5 “*** S ” = 6 , 61
When the image is transferred to a computer, you can refer to the fourth
character of the image file name to identify the image size without opening
the image. RAW or JPEG images can be distinguished with the extension.
If you cannot enter text in step 3, press the < Q > button and use the
character palette when the blue frame appears.
The extension will be “.JPG” for JPEG images, “.CR2” for RAW images,
and “.MOV” or “.MP4” for movies.
When you shoot a movie with User setting2, the file name’s fourth
character will be an underscore “_”.