X3m.System.Scheduler.service_responded

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

service_responded(arg1, state)

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Specs

service_responded(X3m.System.Message.t(), state :: any()) ::
  :ok | {:retry, in_ms :: non_neg_integer(), X3m.System.Message.t()}

This callback is invoked when scheduled message is processed.

It should return either :ok (and remove from persitance) so message delivery is not retried or amount of milliseconds in which delivery will be retried with potentially modifed X3m.System.Message. Its assigns can used to track number of retries for example.