“…A particularly interesting example is the compactifications of 6d N = (2, 0) SCFTs on Riemann surfaces punctured by codimension two defects, resulting in a wide range of 4d N = 2 theories known as class S theories and their dualities [22][23][24]. More recently, the construction is generalized to 6d N = (1, 0) down to 4d N = 1 [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44], and novel connections have been established between elliptic quantum difference equations and surface defects introduced in the 4d theories in terms of their superconformal index [25,41,[45][46][47]. Meanwhile, in the context of 6d N = (1, 0) or their corresponding KK theories, it has been realized that, in the case of rank one 6d theories on the tensor branch with trivial gauge groups, the elliptic quantum difference equations studied in the 4d setup are precisely the elliptic quantum SW-curves of the corresponding 6d theories, and the surface defects in 4d also become a class of important codimension four observables, the Wilson surface defects, that serve the eigenvalues of the quantum curves [48][49][50].…”