Moody’s Unveils Modeling Assumptions for B&C MBS Servicers who stick to their guns when it comes to collecting prepayment penalties help reduce subordination levels on subprime securitization deals. That was one the revelations that emerged from a report, published last week by Moody’s Investors Service, that laid out for the first time the details of the company’s model for determining tranche sizes for most subprime mortgage-backed MBS deals.