The extent of a class (the so-called candidate class) represents the complete set of all persistent instances of that class. In pure object oriented terms, an extent actually contains all classes that are instances of the candidate class, although JDO allows the construction of an extent to optionally include or exclude subclasses of the candidate class.
Extents without subclasses
There are a number of JDO experts, myself included, who believe that the concept of an extent of a candidate class without subclasses does not make sense in terms of object orientation.
We recognize that extents without subclasses do facilitate performance improvements with some Implementations. However, it is our opinion that the subclasses = false setting ...