You define object classes by example. You select an existing object that embodies the object class you are defining (a “model object”). Then you modify the properties for that object to define the class.
Object class definitions are stored in an object class definition file, which you can attach to any map. You can add definitions from multiple drawings to a single definition file. It becomes a library of object classes. You attach the definition file to a map. Then you use the object classes in that definition file to classify existing objects and create new ones.