Abelian class groups of reductive group schemes

We introduce the abelian class group C _ab (G) of a reductive group scheme G over a ring A of arithmetical interest and study some of its basic properties. For example, we show that if the fraction field of A is a global field without real primes, then there exists a surjection C(G) ? C _ab (G), where C(G) is the class set of G.