When the B3 order entry gateway detects a message sequence number gap from the customer and sends the NotApplied message, the Handler calls the OnixS::B3::BOE::SessionListener::onNotApplied method.

The client system uses this method to identify requests that haven't been processed. It is responsible for handling recovery at the application layer, which may involve resending the transactions with new sequence numbers, sending alternative transactions, or taking no action.