You're seeing just the callback ack, go back to Broadway.Acknowledger module for more information.
Link to this callback

ack(ack_ref, successful, failed)

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  ack_ref :: term(),
  successful :: [Broadway.Message.t()],
  failed :: [Broadway.Message.t()]
) :: :ok

Invoked to acknowledge successful and failed messages.

  • ack_ref is a term that uniquely identifies how messages should be grouped and sent for acknowledgement. Imagine you have a scenario where messages are coming from different producers. Broadway will use this information to correctly identify the acknowledger and pass it among with the messages so you can properly communicate with the source of the data for acknowledgement.

  • successful is the list of messages that were successfully processed and published.

  • failed is the list of messages that, for some reason, could not be processed or published.