Is the order of the indexes in ON_3dmObjectAttributes::m_group significant? My experience with Rhino suggests that it is not (i.e. an object’s group list can be treated as an unordered set), but presence of methods like ON_3dmObjectAttributes::TopGroup() and ON_3dmObjectAttributes::RemoveFromTopGroup() suggests that the last item in the list may have special meaning.
The “top” group is just the last group index added to ON_3dmObjectAttributes::m_groups array.
– Dale