A domain object diagram shows the concepts which are to be modeled as objects.
After the conceptual schema has been drawn, a decision about the relative importance of
each concept is made.
Objects are created for the central concepts, and the less important concepts are dropped from the diagram. A domain object diagram shows the relationships between the objects in the domain. One heuristic is to consider all nodes
which have no outgoing arcs to be attributes of the node on the other end of the arc.
Eliminating the attribute nodes from the graph leaves the domain objects. The domain object diagram is basically a subgraph of the entire conceptual schema. No decisions are made in this step other than eliminating attribute nodes.