When the Federal Reserve talks about buying mortgage-backed securities (or MBS), it’s referring to the To-Be-Announced market (usually referred to as the TBA market). The TBA market allows loan originators to take individual loans and turn them into a homogeneous product that can be traded. TBAs settle once a month, and Fannie Mae loans are put into Fannie Mae securities
via Why have Fannie Mae TBAs been underperforming bonds lately? » Market Realist.