A Mortgage Trail Book is the trailing revenue, or rights to this revenue, that a Broker or Advisor or Introducer owns.

When a mortgage is introduced, often the terms of the remuneration include a small ongoing payment based on the ongoing balance of the home loan, generally for the life of that loan.  
This small payment combined with small payments from many other loans can become a significant source of ongoing revenue.
The combined trailing revenue of loans is often referred to a Trail Book.
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