tags or labels

Since Endnote has only a 2-level hierarchical organization and since it is not uncommon for a single reference to be relevant to a number of different categories, is there any way to add “tags” or"labels" to a reference so that you could search for all references in, for instance,  Groupset A, Group B with tag C? Or, for that matter, all references with tag C in Groupset A regardless of which Group they are in?