There are four reasons to create use cases and use case diagrams:

• To discover «requirements» for a system.
• To better understand a system from an external perspective.
• To understand the external and internal factors that influence a system.
• To depict the interaction among the system and the external «actors».

The purpose of this infographic is to depict common characteristics of use case diagrams using the common UML / SysML language.

In this example we are using a passenger airliner.

(Right click over the image and open the image in a new tab.)