Canon EOS R3 Advanced User's Manual page 174

Caution
Expanded ISO speeds (L, H) are not available in HDR shooting.
Some scenes may look different from how they appear on an HDR display device.
Unused signal values are roughly indicated by image areas in gray in the histogram
when [ : HDR shooting ] is set to [Enable].
[ : Disp. performance] is not available when [ : HDR shooting ] is
set to [Enable]. It is set to [Smooth].
For images captured with [ : HDR shooting ] set to [Enable], before
playback on an HDR display device, set [ : HDMI HDR output] to [On] ( ).
Note that regardless of the [ : HDMI HDR output] setting, HDR images are used
for display on HDR display devices.
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