There are cases that a precursor-derived metabolite could be the modification of the dimer, trimer or even polymer of this precursor.
For instance, a metabolite could be [2M+H]+, where M is the precursor. if the mass difference of between labeled and unlabeld precursor is 10 (suppose 10 13C). Then the mass difference between the labeled and unlabeled metabolite will be 20 (2 * 10).
The poly parameter in the diso() function is therefore used to fish out such metabolites. poly = 2 means look for dimer precursor-derived metabolites; poly = 3 for trimer derived metabolites, and so on.