NIMs perform well so far, thanks to high prepayment speeds.(Net Interest Margin)

Standard & Poor's last week revised the way it rates seasoned NIM transactions.

Now, seasoned NIMs - transactions whose underlying collateral have experienced at least five cashflow distributions - will be split into two separate categories. NIMs from deals that have experienced up to six distributions before issuance will be in one category, and those transactions with seven or more distributions will be placed in another. Also qualifying as a seasoned NIM is a re-NIM, a resecuritized residual from a previous NIM deal.

"It is fairly minor, more of a clarification almost," said Terry Osterweil, an analyst at S&P.

Previously, NIMs were considered by …