A relative theory is a boundary condition of a higher-dimensional
topological quantum field theory (TQFT), and carries a non-trivial
defect group formed by mutually non-local defects living in the relative
theory. Prime examples are 6d6d\mathcal{N}=(2,0)𝒩=(2,0)
theories that are boundary conditions of 7d7d
TQFTs, with the defect group arising from surface defects. In this
paper, we study codimension-two defects in
6d6d\mathcal{N}=(2,0)𝒩=(2,0)
theories, and find that the line defects living inside these
codimension-two defects are mutually non-local and hence also form a
defect group. Thus, codimension-two defects in a
6d6d\mathcal{N}=(2,0)𝒩=(2,0)
theory are relative defects living inside a relative theory. These
relative defects provide boundary conditions for topological defects of
the 7d7d
bulk TQFT. A codimension-two defect carrying a non-trivial defect group
acts as an irregular puncture when used in the construction of
4d4d\mathcal{N}=2𝒩=2
Class S theories. The defect group associated to such an irregular
puncture provides extra “trapped” contributions to the 1-form symmetries
of the resulting Class S theories. We determine the defect groups
associated to large classes of both conformal and non-conformal
irregular punctures. Along the way, we discover many new classes of
irregular punctures. A key role in the analysis of defect groups is
played by two different geometric descriptions of the punctures in Type
IIB string theory: one provided by isolated hypersurface singularities
in Calabi-Yau threefolds, and the other provided by ALE fibrations with
monodromies.