Single Source Information: An Agile Practice for Effective Documentation

In agile software development you want to travel as light as possible, and the easiest way to do that is to choose the best artifact to record information. I use the term 'artifact' to refer to any model, document, source code, plan, and so on created during a software development project. Furthermore, you want to record information as few times as possible, ideally only once. For example, if you describe a business rule in a use case, then describe it in detail in a business rule specification, then implement it in code, you have three versions of the same business rule to maintain. It would be far better to record the business rule once, ideally as human-readable but implementable code, and then reference it from any other artifact as appropriate.

Ambler, Scott W. Agile Modeling (2006). Articles>Documentation>Single Sourcing>Agile