X3m.System.Scheduler.save_alarm

You're seeing just the callback save_alarm, go back to X3m.System.Scheduler module for more information.
Link to this callback

save_alarm(arg1, aggregate_id, state)

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Specs

save_alarm(X3m.System.Message.t(), aggregate_id :: String.t(), state :: any()) ::
  :ok | {:ok, X3m.System.Message.t()}

This callback is invoked when X3m.System.Message should be saved as an alarm. Time when it should be dispatched is set in its assigns.dispatch_at as DateTime.

3rd parameter (state) is the one that was set when Scheduler's start_link/1 was called.

If {:ok, X3m.System.Message.t()} is returned, than that message will be dispatched instead of original one. This can be used to inject something in message assigns during save_alarm.