What is a mortgage pair off fee?

Excuse me, could you please elaborate on what exactly is meant by a "mortgage pair off fee"? I'm curious to understand the context in which this term is used and if it's a standard fee associated with obtaining a mortgage or perhaps a specific charge that may vary depending on the lender or the terms of the mortgage agreement. Is it a one-time fee or does it apply recurrently? Any insight you can provide would be greatly appreciated.