ما معنى Prepayment, conditional prepayment rate, PSA benchmark؟

A prepayment is a repayment of mortgage principal ahead of the amortisation schedule: a sale of the house, a refinancing, a partial repayment, or a default bought out of the pool by the guarantor. The conditional prepayment rate (CPR) is the annualised fraction of the balance, after scheduled principal, prepaid in a month; the monthly fraction, the single monthly mortality, is $\mathrm{SMM} = 1 - (1-\mathrm{CPR})^{1/12}$. The PSA benchmark is a standard speed path: 100% PSA is a CPR of 0.2% in a loan’s first month, rising by 0.2% a month to 6% in its thirtieth and constant after; 200% PSA doubles every rate.