The Housing and Economic Recovery Act of 2008 mandates that Fannie Mae and Freddie Mac have a ‘Duty to Serve’ three traditionally underserved markets of:

Rural Housing
Manufactured Housing
Affordable Housing Preservation
The GSEs are tasked with increasing liquidity and investment capital in these markets.

The Federal Housing Finance Agency (FHFA) issued a Proposed Rule on Duty to Serve on December 15, 2015. Comments to the Rule were due on March 17, 2016.