Canon EOS-1D X Mark III Advanced User's Manual page 201

E-TTL II Flash Metering
Set to [Eval (FacePrty)] for flash metering suitable for shots of people.
The continuous shooting speed for high-speed continuous shooting is slower than when
[Evaluative] or [Average] is selected.
Set to [Evaluative] for flash metering that emphasizes firing in continuous shooting.
If [Average] is set, the flash exposure will be averaged for the entire metered scene.
Note
Depending on the scene, flash exposure compensation may be necessary.
Caution
Even when [Eval (FacePrty)] is set, some subjects and shooting conditions may
prevent you from obtaining the expected result.
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