Learn in the textbookDesign and Analysis Methods and Modeling
DefinitionA use case (ユースケース) describes, for each scene in which a user (an actor) uses the system to reach a goal, the interaction between that user and the system.
ExplanationAn actor is not limited to a person and also covers external systems that interact with the system. Because a use case records what can be done as seen from outside rather than how the system processes things inside, it is used when requirements are being organized.
AnalogyIt is like a play script that sets out, scene by scene, the lines the actors exchange, but says nothing about the stage machinery backstage.
Memory hookRead the name literally: a use case is a case of use. You write one for each situation in which the system is used.