It has to do with classifications and helps to determine the ordering of classes and subclasses. If you are defining these from the start, you list some knowns about them to determine which class should be the super class.
A Vechile is used for transportation.
All Cars are Vehicles.
Not all Vehicles are Cars. (You have Buses, Bikes, Motorcycles etc.)
With these facts, you can see that Vehicle should be the superclass and Car would be the subclass of Vehicle.
Hope this helps