There is a set of features, finally, for which the designers can choose among different implementations that are not equivalent, but they have certain advantages and disadvantages. There are plenty of programming models (C++, Lisp, Smalltalk, etc.), but none of them should be considered better than the others. The designers choose the programming model of their system according to the kind of applications that the system is going to serve. The choice of the programming style is open as well. The one that better suits the applications should be chosen. The representation system is the set of the types or classes provided by the system as well as the set of constructors that can be applied on these classes. As long as the system provides support for extensibility and composite objects, there is no restriction of which member the representation should contain. There are systems that support the highest degree of uniformity, which means that everything in the system including classes, methods, messages, etc. is treated as an object. Uniformity has consequences at the level of the implementation of the system and at the level of the application programming and the user interface as well. Although uniformity is a nice feature and simplifies the implementation of the system, it can sometimes confuse the users since in reality there is no absolute uniformity.

There are many applications that have been using the relational systems very successfully now for many years and they do not need to change. However, there are a couple of other applications especially in the engineering fields that don’t do much with relational systems, mainly from the modeling aspect. For these kinds of applications, the object-oriented approach seems quite appropriate in spite of the problems that still have to be solved.