The ZOO system demonstrates that it is possible to design a graphical language for representing both simple and conceptual objects of an application domain. The usage of icons makes this language easy to understand. Many people are already using graphic diagrams for the specification of technical systems. A ZOO diagram is more than a specification, it is a piece of executable software ready to run immediately after its construction. Moreover, ZOO processes active knowledge bases rather than passive text files, therefore the usage of the system may be supported by the knowledge already existing in the system.
Manipulating the concepts of an application system by using the ZOO knowledge editor corresponds to defining data in conventional data base models. Since meta-level knowledge and object-level knowledge are represented basically the same way, the processes of data manipulation and data definition are handled uniformly. The data manipulation language and the data definition language are identical and may both be represented in a graphical form. The conceptual model which defines the structure of the data is itself part of the data base and may be accessed by the user. The uniform representation of object-level knowledge and meta-level knowledge is therefore of high importance for the construction of user interfaces which help the user to understand and to modify the behavior of a software system.