It's practically identical to othr areas of "software". If you buy a Relational Database Management system (e.g. Access with developers kit) you're usually allowed to design practically anything with it and sell on - except a Relational Database Management System. You can usually sell on anything you write in a computer language (such as, say, visual Basic) EXCEPT for a competing computer language.. and so on and so forth. There are, of course, "grey areas" in any area of life - but the rules pretty much have to be black-and-white to be halfway comprehensible.