Service Model
A Service Model could be one of the more abstract terms used in management packs. You could see a service model as a description of the application or service you are trying to monitor. A logical representation of sorts. The service model basically defines the parts and components and their relations of the application to are building your management pack for. You should have a service model defined before even starting to build a new management pack as it will make it much, much easier to actually create your classes and relationships.
When designing the Service Model you are very likely to create to many classes for a start. My general advice is to make sure that every single one of your classes actually fills a purpose. If you find that when you start plotting your Health Model that you end up with classes that does nothing and really has no real use you are probably better off removing them entirely.
A word of advice; keep it as simple as possible! Do not try to make it completely true to reality, you will never finish it. Aim for “good enough”.
