The subclassPersistentAllowed class option enables you to set the persistentAllowed and subclassPersistentAllowed class option values for all subclasses of this class.
When the subclassPersistentAllowed option is set to false on a superclass by unchecking the Allow Persistent Subclass Instances check box on the Lifetime sheet of the Define Class dialog, the Allow Persistent Instances and Allow Persistent Subclass Instances check boxes are disabled on the Lifetime sheet of the Define Class dialog for all subclasses of the current class so that you cannot change these values. (For details, see "Adding Classes to Your Schema", in Chapter 3.)
An exception is raised at every attempt to create a persistent instance of a subclass. If persistent instances of subclasses exist, you must delete these instances before making the change.