Canon EOS-1D X Mark III Advanced User's Manual page 202

Continuous Flash Control
Set to [E-TTL each shot] to perform flash metering for each shot. Set to [E-TTL 1st shot] to
perform flash metering for only the first shot before continuous shooting. The flash output
level for the first shot is applied to all subsequent shots. Useful when prioritizing continuous
shooting speed without recomposing shots.
Caution
Any subject movement in continuous shooting may cause exposure problems.
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