Extension classes are branches of any other class. Extension classes share attributes and can share form associations and folder associations. Extension classes keep a relationship to the class they were based upon. You can add additional attributes to the extension class. These attributes will only be available in the extended class. When you add an attribute to the class from which an extension class was extended, that attribute will be available in any subsequent extended class. Data that is added to a base class will be appropriately reflected in all extension classes configured for the base class. Likewise, data that is added to an extension class will be appropriately reflected in the base class and all of its extension classes.
To create an extension class: