It seems likely that having a very
elevated aggregate capacity was helpful in controlling market rates initially, perhaps because it showed the FOMC's commitment to achieving interest rate control, but it's unclear exactly how much available capacity, or «headroom,» is needed to maintain such control.
As the FOMC made clear in the minutes to its March 2015 meeting, this
elevated capacity was intended to ensure a smooth liftoff, but the absence of an
aggregate cap is only temporary.