Canon EOS-1D X Mark III Advanced User's Manual page 165

Note
In the viewfinder, maximum display of the number of shots available is 1999.
is set automatically if you set both RAW and JPEG/HEIF to [ ].
Two versions of each shot are recorded at your specified image quality when you
have selected both RAW and JPEG/HEIF. Both images have the same file number
but each has a different file extension, with .JPG for JPEG, .HIF for HEIF and .CR3
for RAW.
< > / < > appears at the right in the viewfinder based on your JPEG/RAW
selection.
Meanings of image size icons are as follows: : RAW, : Compact RAW,
JPEG: JPEG, HEIF: HEIF, : Large, : Medium 1, : Medium 2, : Small.
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