This is an example of UML sequence diagram for online book shopping process. From this diagram, people should search and select the book they want, then checkout. A sequence diagram is a form of interaction diagram because it illustrates how (and in what order) a collection of items interact with one another. Software engineers and business experts use these diagrams to understand the requirements for a new system or to describe an existing process. Event diagrams and event scenarios are other names for sequence diagrams.