When the WRX came to the U.S. in the 2002 model year, some fans were disappointed with the brakes the U.S. cars were equipped with. Instead of the fancy, and cool-looking four-piston (front) and two-piston (rear) fixed calipers the Japanese market car got, the U.S. cars had cheaper and heavier sliding calipers with two pistons in front and a single piston in back. (The word "pot" is shorthand for piston, in case you were wondering.)
With the exception of one shining moment from 2006 and 2007, the WRX has stuck with the cheapy sliding calipers, even up through the 2016 model year.
Turns out the four-pot/two pot brakes will bolt on to any other WRX model, with the help of some special brackets and rotors, so we developed kits for just that purpose. We've got a kit for the 2002-2005 WRX, a kit for the 2008-2014 WRX, and one for the 2015+ WRX. Note that the 2015+ cars finally went to a larger front rotor, so if you are going to bolt on the four-pot front brakes, you'll have to downsize the front rotor to fit.
Note that these aren't the biggest or best brakes Subaru ever fitted to a car. That honor goes to the STi Brembos.