Event-triggered break start (spec 1.1.0): the break is scheduled by a playback event, not a time.
Optional
Seconds after the event before the break triggers. For start this counts played media time only: pausing or seeking does not advance it.
start
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Typed as BreakEvent, but unknown future values may arrive (forward compatibility).
Event-triggered break start (spec 1.1.0): the break is scheduled by a playback event, not a time.