One of the most common things people want to be able to do is extend the model to create new classes, usually new configuration items like customers and other physical assets like phones, desks, etc. Here is a quick demo video showing you how to do this.
This demo video shows you how to create a new configuration item class using the Service Manager authoring tool. The new configuration item class used for the example is a Customer class. The new class will have a key automatically incrementing string property, a customer name string property. a contract start datetime property and a customer type list data type property to show the different kinds of properties that can be added. Then you will see how you can immediately start using the new class by leveraging the generic form and how you can create a custom form if you want to control the look/feel of the form.
If your screen resolution is high enough I recommend watching this full screen at 720p HD.
BTW - there is also a great blog post here from one of the Microsoft partner systems integrators here: