  Any four-dimensional Supersymmetric Quantum Field Theory with eight
supercharges can be associated to a monoidal category of BPS line defects. Any
Coulomb vacuum of such a theory can be conjecturally associated to an "algebra
of BPS particles'', exemplified by certain Cohomological Hall Algebras. We
conjecture the existence of a monoidal functor from the category of line
defects to a certain category of bimodules for the BPS Algebra in any Coulomb
vacuum. We describe images of simple objects under the conjectural functor and
study their monoidal structure in examples. As we vary the choice of vacuum, we
expect the collection of functors associated to any given theory to capture the
full information of the original monoidal category of lines.
