test-driven development Questions

Should stories be kept in place of requirements to provide permanent system documentation or should a functionally organized set of requirements be maintained as this documentation.     Either way, I am trying to figure out the best way to maintain a description/model of the system being maintained that is understandable to non technical and technical staff and provides a starting point for analyzing future changes to the system.

Can You guys tell me the current trends /techniques that are followed in the test case Designing 

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