You're seeing just the function pdus, go back to SMPPEX.ESME.Sync module for more information.
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pdus(esme, timeout \\ 5000)

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pdus(esme :: pid(), timeout()) :: [awaited()]

A nonblocking version of wait_for_pdus/2.

The difference is that it always immediately returns a list of items(maybe empty) and never returns :timeout or :stop.