Canon EOS R5 Advanced User's Manual page 357

Note
The maximum brightness value does not reach 100% when HDR-PQ is set. Note
that the maximum brightness value varies depending on [ : Highlight tone
priority] and [ : Picture Style] settings.
The maximum brightness value does not reach 100% when Canon Log or an
expanded ISO speed is set.
Checking the zebra display level in advance is recommended when you will set
[Zebra pattern].
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