Timber, gave or lent at near zero interest rate, where the banks could use this to buy bonds ahead of Fed purchases and then onsell them for a profit.
$16T is no mean thing considering the GDP of the US is less than that. The size of the U.S. Stock Market is about $14T and even the huge USBond market comes in at about $35T.
BTW did you see jobs report today, the delayed December nonfarm payroll report (74K actual v. 197K expected) that saw the unemployment rate drop to 6.7% (7.0% expected). Will this slow down taper? Bond and gold market think it may.