public Extent getExtent (Class pcClass, boolean includeSubclasses);
Extents are logical representations of all persistent instances of a given persistence-capable class, possibly including subclasses.
Extents are obtained through the PersistenceManager's getExtent method. This method takes two parameters: the class of objects the Extent contains, and a boolean indicating whether or not subclasses are included as well.
You cannot retrieve Extents for persistence-capable classes whose metadata specifies a value of false for the requires-extent attribute.