What does the resume keyword add to an NPL notification?

resume attaches a callback to a notification, so that after the external work is done, control comes back into the protocol via a named permission.

Often, firing an event is not the end of the story — a follow-up action must happen once the outside world responds. NPL supports this by letting the notification carry callback information. You declare it to return a callback argument type:

notification PayInterests(iban: Text, amount: Number) returns TypeOfCallbackArgument;

and fire it with a resume target:

notify PayInterests(iban, interests) resume userAction;

Here userAction is a permission defined on the protocol that sent the notification. When the external system finishes and calls back, the Runtime invokes userAction to continue the workflow. This closes the loop: fire an event out, and route the response back into the same protocol as a real state-changing action.