Models, Atoms, References, Connections and Sets are all first class objects, or FCO-s for short. Atoms are the elementary objects - they cannot contain parts. Each kind of Atom is associated with an icon and can have a predefined set of attributes. The attribute values are user changeable. A good example for an Atom is an AND or XOR gate in a gate level digital circuit model.
Models are the compound objects in our framework.
They can have parts and inner structure. A part in a container Model always has a Role. This containment relationship creates the hierarchical decomposition of Models. Any object must have at most one parent, and that parent must be a Model. Aspects provide primarily visibility control.