This Class and Package Diagram depicts the notions of a system (or the notions supported by a system), as well as their properties and inter-relationships. Classes represent the concepts supported by a system at the highest level (conceptual), while at the lowest rank (physical), they can represent the classes implemented by an object language. The examples below demonstrate using class/package diagrams to represent some basic systems. The following illustration depicts a conceptual view of Human society. This is a typical application of class diagrams, with classes, generalizations, associations, and an operation. Some model elements also have constraints attached to them.