In one of the talks of J.P. May he mentioned some examples of structure groups and their classification spaces (he mentioned: O, U, SO, SU, Sp, Spin, String, Top, STop, F and SF). Most of them are familiar with me, but I never heard of the structure group String. Googling isn't helping much, because you always find yourself reading some physics article. The question is simple:
- What does the group $String$ mean in the context of fibre bundles?
- In what articles / books are they explained ?