"Regional Rally Cars" are S2000 1.6T. The former is an unofficial term, the latter is the name used by FIA.
Note that since 2011, S2000 cars can be homologated with 1.6T engines only and current WRCs are S2000 1.6T cars with detachable WRC-kit (wing, flywheel, etc.). Therefore, every WRC (of the current generation) is a "RRC" with an additional kit.
Moreover, if a company wants to homologate a WRC car, they need to show FIA ten S2000 1.6T cars (or parts for those cars) plus ten WRC-kits, thus "RRC" and WRC are basically the same cars, and nowadays no company will build a S2000 if their intention is other than to participate in World Rally Championship. Therefore, FIA were forced to introduce R5 (low-cost cars, with production based 1.6T engines) to allow companies like Skoda, with no intention of going into WRC, to homologate new cars.
To summarize, the "RRCs" are currently limited to Citroen DS3, Ford Fiesta, Mini Cooper and VW Polo in the near future. The next addition will be i20, if Hyundai's WRC-plans will come to fruition.
I hope this clarifies the issue
