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  • Alex Kim thinks this is interesting:

The GSEs also buy loans for cash proceeds through what is called, appropriately enough, the cash window. This is often used for loan programs with unusual specifications such as certain documentation styles or loan-to-value ratios, as well as by smaller lenders that engage in piecemeal sales. Loans purchased through the cash window can either be securitized in multi-issuer pools or retained in the GSEs' portfolios.

From

Cover of Mortgage-Backed Securities: Products, Structuring, and Analytical Techniques, 2nd Edition

Note

not quite familiar with this