does it mean - paying off the entire remaining debt; OR - prepayments for less than the outstanding principle balance (as per Schweser book 5 p112)?
I think it can mean either. It is any payment in excess of the required payment that shortens the life of the mortgage.
curtailment is paying less than the entire amount… something more than regular payment and less than the entire amount. So essentially you are reducing the overall time for which the “guarantor/issuer” would be receiving interest for, hence causing cash flow related issues. only form of entire repayment to an original loan servicer - is a refinancing when you (loan holder) now start payments at a newer lower rate to a new loan servicer. and this refinancing happens when interest rates fall.