The percentage of the monthly cash flow that remains after the cash flow has been divided into a coupon and principal payment for the mortgage backed securities (MBS) holder. This servicing fee typically goes to the servicer of the loan, and is possibly a guarantee fee for the underwriter of the MBS.  



|||For example, in a typical MBS deal, if the interest rate on a mortgage is 8%, the MBS holder might receive 7.5%, the servicer of the mortgage receives 0.25% and the MBS underwriter gets 0.15%. This leaves the reamining 0.10% (8% - 7.5% - 0.25% - 0.15% = 0.10%) as excess servicing.

Like an MBS, excess servicing is subject to prepayment and extension risk. When excess servicing is priced, it is valued based on an estimate of how long the annuity will last. This must be estimated since it cannot be known for certain when a mortgage borrower might refinance or otherwise pay-off his or her mortgage. The value of excess servicing can change dramatically when interest rates change, because changes in current interest rates relative to the interest rate on the mortgage determine how long the annuity of excess servicing associated with that mortgage might last.