  Starting with a choice of gauge algebras, specification of a 4d gauge theory
involves additional data, namely the gauge groups and the discrete theta
angles. Equivalently, one needs to specify the set of charges of allowed line
operators. In this note, we study how these additional data are represented in
6d, when the 4d theory in question is an N=4 super Yang-Mills theory or an N=2
class S theory. We will see that the Z_N symmetry of the so-called T_N theory
plays an important role. As a byproduct, we will find that the superconformal
index of class S theories can be refined so that it can give 2d q-deformed
Yang-Mills theory with different gauge groups associated to the same gauge
algebra.
