Canon EOS-1D X Mark III EOS-1DX Mark III Advanced User Guide - Page 220

Caution JPEG/HEIF images cannot be selected. These images cannot be selected as the first single exposure: images shot with [ : Highlight tone priority] set to [Enable] or [Enhanced], or images with cropping information. [Disable] is applied for [ : Lens aberration correction] and [ : Auto Lighting Optimizer], regardless of the settings of the image selected as the first single exposure. The ISO speed, Picture Style, high ISO speed noise reduction, color space, etc. set for the first image will also be applied for the subsequent images. If Picture Style is [Auto] for the image selected as the first image, [Standard] will be applied for shooting. You cannot select an image taken with another camera. Images that cannot be merged may be displayed. Note You can also select a RAW image used in multiple-exposure shooting. Select [Deselect img] to cancel image selection. 220