Hi Suresh,

An abstract class is a rule (an instance of the Rule-Obj-Class class) created to support the definition of rules, including other classes. Such rules can be inherited by subclasses of the abstract class. Rules with a class as a key part (such as properties, activities, flows, data transforms, and so on) can apply to an abstract class.
A class that is not abstract is concrete. Unlike abstract classes, concrete classes can have instances stored in the database. A selection on the Class rule form determines whether a new class is abstract or concrete.
An abstract class can be a child of a higher abstract class.

A selection on the Class form determines whether a class is concrete or abstract.

Concrete classes usually appear near or at the bottom (leaf nodes) of the class hierarchy, but may appear anywhere below the top level.

Hope this helps!

Reagrds,

Gisela