Instantiability in structured types

Types can also be defined to be INSTANTIABLE or NOT INSTANTIABLE.

By default, types are instantiable, which means that an instance of that object can be created. Conversely, noninstantiable types serve as models intended for further refinement in the type hierarchy. For example, if you define Person_t using the NOT INSTANTIABLE clause, then you cannot store any instances of a person in the database and you cannot create a table or view using Person_t. Instead, you can only store instances of Employee_t or other subtypes of Person_t that you define.