If you're replacing an old system, a great piece of documentation is the original requirements against which it was built, especially if that document is versio

If you're replacing an old system, a great piece of documentation is the original requirements against which it was built, especially if that document is versioned. Goes a long way to understanding why something was built a certain way and whether the new thing can do the job of the old thing. It's so much better than something either overly vague or overly specific like "it must work (exactly) the same way as the old system".