You're seeing just the callback handle_send_pdu_result, go back to SMPPEX.Session module for more information.
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handle_send_pdu_result(pdu, send_pdu_result, state)

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handle_send_pdu_result(pdu :: SMPPEX.Pdu.t(), send_pdu_result(), state()) ::

Invoked when the SMPP session successfully sent PDU to transport or failed to do this.

pdu argument contains the PDU for which send status is reported. send_pdu_result can be either :ok or {:error, reason}.

The returned value is used as the new state.