The representation of is-a relationships. An is-a relationship is a binary relationship that is an inclusion function. Extensionally, the set of all possible cars is a subset of the set of all possible vehicles. Intensionally, the set of properties shared by all cars includes the set of properties shared by all vehicles. A generalization can be specialized by any number of specialization groups.
Cars are vehicles and trucks are vehicles.
The union of the set of all cars and all trucks equals the set of all vehicles. Diesel vehicles are vehicles and gas vehicles are vehicles. There is no vehicle both a diesel and a gas vehicle. There may be vehicles that are neither diesel nor gas vehicles. The CRD representation of relationships.