They are an exploratory way of learning the system and answering the question, 'Is there a problem here?'
测试是了解系统和解答问题的探索之旅：“这里有问题吗？”
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No question migrating birds are answering a command of nature, but now the bad news: There is growing evidence that the wiring is off; the system is breaking down.
Using Traceability Matrices, you can query on use cases not yet traced to test requirements (answering the question How do you know that the entire system functionality is tested?).