Actually, it's not. There are literally dozens of variations of the Fiesta rally car - the RS WRC, S2000, RRC, R5, R34, R3T, R2 and so on - with only minor differences between them to comply with each of the classifications of rally car. Of all the variations, the R5 has the most in common with its parent model, the Fiesta RS WRC. It was built by M-Sport as a prototype of the Group R regulations that were introduced in 2012.
R5 is suitable for everything up to WRC-2, and it is widely believed that it will replace the WRC from 2017. Rallying is currently a mess of dozens of categories, so the entire system is being overhauled and replaced by Group R. If the R5+ was really that different to the R5, it would be reclassified as R3T.