What is a node class?

What is a node class?
A node class is a class that,
* relies on the base class for services and implementation,
* provides a wider interface to the users than its base class,
* relies primarily on virtual functions in its public interface
* depends on all its direct and indirect base class
* can be understood only in the context of the base class
* can be used as base for further derivation
* can be used to create objects.
A node class is a class that has added new services or functionality beyond the services inherited from its base class.