Lets say I want to build a car. There are national and international standards regarding quality and specifications what I should follow, which say,for example, that I have to have brakes in my car. So logically I suppose it is obligatory to follow standard and without it they wouldn't let me build this car for clients. But is it so? Remember from lectures professor saying that standards are optional...but it doesn't make sense then... Can someone clarify me who provides must-follow rules, and who provides optional guidelines ?