Why circle aircraft and not the names of the aircraft in the fleet?  It is because 747 and DC-10 are specific types of the generic term aircraft. The type of aircraft is an attribute of the generic aircraft entity. Several cargo shipments and flight plans can exist. Like the specific aircraft, these are instances of their respective generic terms. For the Vocabulary, you identify the generic (and therefore reusable) terms.  But, ultimately, you need a way to identify specific cargo shipments and flight plans from within the set of all cargo shipments and flight plans. Assigning values to attributes of a generic entity accomplishes this goal, discussed later.