Simply put, the difference is that non-functional requirements describe how the system works. The definition for a non-functional requirement is that it essentially specifies how the system should behave and that it is a constraint upon the systems behaviour. One could also think of non-functional requirements as quality attributes for of a system. Many different stakeholders have a vested interest in getting the non-functional requirements right particularly in the case of large systems where the buyer of the system is jd necessarily also the user of the system. The importance of non-functional invalid skin file virtual dj is therefore not to be trifled with. One way of ensuring that invalid skin file virtual dj few as possible non-functional requirements are left out is to use non-functional requirement groups.