Canon EOS-1D X Mark III Advanced User's Manual page 262

Note
Notes for < >, < >, and < > modes
You can lock the exposure (AE lock) by pressing the < > button. After applying
AE lock during movie recording, you can cancel it by pressing the < > button.
(AE lock setting is retained until you press the < > button.)
Exposure compensation can be set in a range of up to ±3 stops by turning the
< > dial.
In < > mode, the ISO speed, shutter speed, and aperture value are not
recorded in movie Exif information.
The camera is compatible with Speedlite functionality to activate the LED light
automatically under low light when recording movies in < >, < >, and
< > modes. For details, refer to the Instruction Manual of the EX series
Speedlite equipped with an LED light.
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