The aggregation diagram describes domain objects which play an attribute role in the conceptual diagram but must be modeled as separate objects because they have their own attributes. For example, an experiment has a relationship with a filter, a probe, and an investigator, and an experiment cannot exist in the database without probe and filter. Therefore an experiment is an aggregation of probe and filter. The same experiment could be described without knowing the person who investigated it, so the person is not part of the aggregation.