EventStore.ack

You're seeing just the callback ack, go back to EventStore module for more information.

ack(subscription, arg2)

Specs

ack(
  subscription :: pid(),
  EventStore.RecordedEvent.t()
  | [EventStore.RecordedEvent.t()]
  | non_neg_integer()
) :: :ok | {:error, reason :: term()}

Acknowledge receipt of the given events received from a subscription.

Accepts a single EventStore.RecordedEvent struct, a list of EventStore.RecordedEvents, or the event number of the recorded event to acknowledge.