Extreme.connect_to_persistent_subscription

You're seeing just the callback connect_to_persistent_subscription, go back to Extreme module for more information.
Link to this callback

connect_to_persistent_subscription(subscriber, stream, group, allowed_in_flight_messages)

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Specs

connect_to_persistent_subscription(
  subscriber :: pid(),
  stream :: String.t(),
  group :: String.t(),
  allowed_in_flight_messages :: integer()
) :: {:ok, pid()}

Spawns a persistent subscription.

The persistent subscription will send events to the subscriber process in the form of GenServer.cast/2s in the shape of {:on_event, event, correlation_id}.

See Extreme.PersistentSubscription for full details.