A system use case diagram displays the relationships between consumers and providers of application services. Application services are consumed by actors or other application services and the application use case diagram provides added richness in describing application functionality by illustrating how and when that functionality is used. The purpose of the system use case diagram is to help to describe and validate the interaction between actors and their roles with applications. As the architecture progresses, the use case can evolve from functional information to include technical realization details. Architectural system use cases can also be re-used in more detailed system design work.