The specialization/generalization of the Staff entity is optional and disjoint (shown as
{Optional, Or}), as not all members of staff are Managers or Supervisors, and in addition
a single member of staff cannot be both a Manager and a Supervisor. This representation
is particularly useful for displaying the shared attributes associated with these sub classes and the Staff super class and also the distinct relationships associated with each subclass,namely Manager Manages Branch and Supervisor Supervises Staff.