6.3. Checking Associations
The multiplicities of associations must be checked to ensure a complete and correct understanding of the relationship in the domain. To do so, consider the number of possible instances that can participate in the relationship.
As an example, consider the association between senators and senate seats. Each state in the United States has two senate seats, each of which can be occupied by one person at a time. However, a senate seat could be unoccupied due to the death or resignation of a senator. Hence, although a senate seat is normally occupied, it is not required that a senate seat is always occupied.
The multiplicity of the association between a Senate Seat and a Senator at the Senator end is ...