 This subset is still too big, because some of these classes are related by large gauge transformations.In QED there are large gauge transformations which add integral multiples of two pi to Wilson loops.The p-form potentials in type II supergravity theories also enjoy these large gauge transformations, but due to the presence of Chern-Simons terms in the supergravity actions these large gauge transformations transform not only the p-form potentials but also simultaneously the (p+3)-form field strengths.