a use case captures......翻译成中文

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  A use case captures a contract between the stakeholders of a system about its behavior
The use case describes the system’s behavior under various conditions as it responds to a
request from one of the stakeholders, called the primary actor .
  A Use Case is a collection of related success and failure scenarios that describe actors using a
system to fulfill (or support) a goal.
  A scenario is a specific sequence of actions and interactions between actors and the system under discussion.