Optional.
This ID can be any unique string chosen by the advertiser. The event_id and event_name parameters are used to deduplicate events sent by both web (via the Meta Pixel) or app (via SDK or App Events API) and the Conversions API. Note that while event_id is marked optional, it is recommended for event deduplication.
For deduplication, the eventID from a browser or app event must match the event_id in the corresponding server event. Learn more about Handling Duplicate Pixel and Conversions API Events.
An order number or transaction ID are two potential identifiers that can be used for event_id. For example, if a customer makes two purchases on your website with order numbers 123 and 456, each Conversions API call would need to include its respective order number for event_id. This allows us to properly distinguish these two purchase events as distinct orders. The two corresponding browser Pixel purchase events would need to also send the same order numbers in the eventID parameter for us to understand that there were only two events that took place, not four unique purchases.
For other events without an intrinsic ID number, a random number (so long as the same random number is sent between browser and server events) can be used.
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