Representing all kinds of objects, relationships and processes using a single diagram would be too complex and confusing. For this reason, 14 different types of diagrams are used in the UML, which can be divided into structure diagrams, behavior diagrams, and interaction diagrams . Interaction diagrams, in turn, are a subcategory of behavior diagrams.
The structure diagrams focus on representing all system elements and their relationships with each other. A typical example is the class diagram, with which elements can be grouped and displayed in hierarchies. The behavior diagrams , however, do not represent static structures, but show the flow of the planned or actual process that should take place in the run the program or software. In this type of diagram, the dynamics take center stage..
The use case diagram also belongs to the latter group, but it is a particular model , because it shows the expected behavior of a system or software in a specific use case . Compared to other behavior diagrams in the UML, the use case diagram is fairly static, as it can only be used to describe actions and objectives, but not the exact sequence of processes and actions. For the latter, other types of diagrams are used in UML, such as activity diagrams, which represent the processes chronologically; or the sequence diagrams, which show the exchange of messages between the different elements that make up a system.