Raccomando di leggere Joel sul software. Non sono sicuro che risponda alle tue domande specifiche, ma ha una eccellente panoramica di cosa significa scrivere le specifiche funzionali :
The most important function of a spec is to design the program. Even if you are working on code all by yourself, and you write a spec solely for your own benefit, the act of writing the spec — describing how the program works in minute detail — will force you to actually design the program...
...when you design your product in a human language, it only takes a few minutes to try thinking about several possibilities, revising, and improving your design. Nobody feels bad when they delete a paragraph in a word processor. But when you design your product in a programming language, it takes weeks to do iterative designs. What’s worse, a programmer who’s just spend 2 weeks writing some code is going to be quite attached to that code, no matter how wrong it is...
...When you write a spec, you only have to communicate how the program is supposed to work once. Everybody on the team can just read the spec. The QA people read it so that they know how the program is supposed to work and they know what to test for. The marketing people use it to write their vague vaporware white papers to throw up on the web site about products that haven’t been created yet. The business development people misread it to spin weird fantasies about how the product will cure baldness and warts and stuff, but it gets investors, so that’s OK. The developers read it so that they know what code to write. The customers read it to make sure the developers are building a product that they would want to pay for. The technical writers read it and write a nice manual...
When you don’t have a spec, all this communication still happens, because it has to, but it happens ad hoc. The QA people fool around with the program willy-nilly, and when something looks odd, they go and interrupt the programmers yet again to ask them another stupid question about how the thing is supposed to work...
without a detailed spec, it’s impossible to make a schedule... In too many programming organizations, every time there’s a design debate, nobody ever manages to make a decision, usually for political reasons. So the programmers only work on uncontroversial stuff. As time goes on, all the hard decisions are pushed to the end... Writing a spec is a great way to nail down all those irritating design decisions, large and small, that get covered up if you don’t have a spec...