So aggregate is now called shared aggregation as both can exist independently.
Composite is now called composite aggregation. The part can’t exist apart from the whole.
Compositional containment hierarchy Then has both types.
Crucially to me there is a pluralism and an conditional independence here. Within the levels there is autonomy but there is also sometimes interaction between levels.
Are the higher levels required? Not necessarily. So perhaps it’s mostly shared aggregation? I suppose each system would need to be labeled as shared aggregation, composite aggregation or both.
Haven’t even gotten to sibling relationships but somehow I think that’ll be easier.