In the previous paragraphs we have added a few of the classes we need to model the PBX. Let us consider briefly adding some classes that we do not need. Take, for example, our power supply. The common models contain a very adequate class with many properties called CIM_PowerSupply which inherits from CIM_LogicalDevice which is precisely where we would introduce our ACNE_PBXPowerSupply. Do we need to define our own class here?
The strict answer is "no"; we can use the one already defined. There might, however, be advantages later in defining a ACNE_PBXPowerSupply class which inherits from CIM_PowerSupply and adds no new features. This would give us flexibility in the future if we did find that our power supply has a property or feature not already modelled.
This is a judgment call that you would need to make on a case by case basis. In this case I believe that I would feel more comfortable with an additional class. For the purposes of this exercise, however, I will not add any unnecessary classes.