One object must be common to all relationships in the collection. That object must play the same role (either origin or destination) for all relationships in the collection. In the relationship collection shown in the following figure, every relationship uses the object describing Mike as the performer of the work on a subproject. The object describing Mike is the origin object in this relationship.
In contrast, the set of relationships shown in the following figure does not constitute a collection, because there is no object that all the relationships use in the same role. In fact, the relationships have no object in common, regardless of role.
