We investigate a class of generating series which enumerate multi-analogues of set partitions with very general weights and constraints imposed, and develop some of the relevant theory. The weights and constraints we consider are embodied in the definition of a “system,” which includes weighted multiset systems as a simple special case. Three topics are discussed. First, we derive a composition formula valid for all systems, which specializes to composition formulas for familiar combinatorial structures in many cases. Second, we extend the Heilmann–Lieb theorem on matching polynomials to a similar statement valid for more general factors of multigraphs. Finally, we introduce a multi-analogue of the order polynomial of a labelled poset, and by applying our general composition theorem give a formula for the effect of composition of labelled posets on their E-polynomials.