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An Aggregation relationship is a type of association that shows that an element contains or is composed of other elements. It is used in Class models, Package models and Object models to show how more complex elements (aggregates) are built from a collection of simpler elements (component parts; eg. a car from wheels, tires, motor and so on).

A stronger form of aggregation, known as Composite Aggregation, is used to indicate ownership of the whole over its parts. The part can belong to only one Composite Aggregation at a time. If the composite is deleted, all of its parts are deleted with it.

After drawing an Aggregation association, its form can be changed.

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