We use the F-theory realization of 6D superconformal field theories (SCFTs)
to study the corresponding spectrum of stringlike, i.e. surface defects. On the
tensor branch, all of the stringlike excitations pick up a finite tension, and
there is a corresponding lattice of string charges, as well as a dual lattice
of charges for the surface defects. The defect group is data intrinsic to the
SCFT and measures the surface defect charges which are not screened by
dynamical strings. When non-trivial, it indicates that the associated theory
has a partition vector rather than a partition function. We compute the defect
group for all known 6D SCFTs, and find that it is just the abelianization of
the discrete subgroup of U(2) which appears in the classification of 6D SCFTs
realized in F-theory. We also explain how the defect group specifies defining
data in the compactification of a (1,0) SCFT.Comment: 24 page