Aggregation is a form of association in which one element consists of other, smaller constituents. This relationship is depicted by a diamond-shaped white arrowhead. In this case, if the parent object is deleted, the child object may still continue to exist.
Figure 12 shows an aggregation relation between an insurance agent and the local insurance office in which he works. The local insurance office is where insurance agents carry out tasks such as policy underwriting, depositing premiums for their customers, and various other functions. So even if the local office is closed down, the agent can report to another office. Similarly, the agent can de-register from a local office and move to a different office of the same insurer.
Figure 12. Aggregation
Composition is a stronger form of aggregation; as in this case, if the parent is deleted, the children will also no longer exist. This relationship is depicted by a diamond-shaped solid arrowhead.
Figure 13 shows the composition relationship between a party involved in some policy or claim and their address. If the party is deleted from the system, its address will also be deleted.
Figure 13. Composition