The RS series are limited production racebikes. They are factory tuned and the engines are not built by the "normal" Ducati employees but by the Corsa racedepartment at the factory.
RS-series bikes generally have VERY aggressive cams and are VERY service intensive. I´d not recommend one of those for the street, since valve adjustment drops to like every 1000 miles MAX.
But as a trackweapon they are pretty nice. Since the bike is purpose built for racing, It´s got all the lighter parts from the factory already, aswell as Öhlins suspension and a tuned engine, QC-sprocket carriers and so on..
The RS badge has been put on 748´s, 916´s, 996´s and 998´s
I´m not sure about newer and older models.. I think they where called SPS/R