Polygroup sets can be stored in multiple places. The default location is in the per-triangle group integer stored directly on a FDynamicMesh3. Additional layers may be stored in the FDynamicMeshAttributeSet. Future iterations could store packed polygroups in other places, store them in separate arrays, and so on. FPolygroupSet can be used to abstract these different cases, by providing a standard Polygroup Get/Set API.