Canon EOS R3 Advanced User's Manual page 201

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.
[E-TTL 1st shot] is set when [
[Mechanical] in [
: Shutter mode] is set to [Elec. 1st-curtain] or
] drive mode.
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