Below is a numbered code snippet which will answer both the upper questions. Lets understand in detail the numbered sections.
1 and 2 -- This defines the interface definition. Function.abstractMethod() defines a method as abstract.
3 - We need to register the interface which is done by using registerInterface method.
4, 5 and 6 - Inheritance and Interface is defined when we register the class. registerClass has three inputs. 4th section defines the main class which needs to be registered. 5th section defines the parent class from which it will inherit. 6th section defines the interface.
So clsMyCustomCustomer is the class which derives from clsCustomer and implements ICustomer interface.