09-06-2017, 03:25 PM
Functional requirements specify what the product should do. They refer to the actions that the product must carry out to satisfy the fundamental reasons of its existence. Think about functional requirements such as business requirements. That is, if you talk to a user or one of the business people, they describe the things the product must do to complete a part of their work. Keep in mind that the requirements specification will become a product contract to be built. Therefore, the functional requirements must fully describe the actions that the desired product can perform. I also relate it to a product I could buy in a store - if you look at the list of bullet features on the back of the box, it describes the functionality of the product.
Non-functional requirements are the properties that your product should have. Think of these properties as the characteristics or qualities that make the product attractive, or usable, or fast, or reliable. These properties are not necessary because they are fundamental activities of the product: activities such as calculations, manipulation of data, etc., but they exist because the client wants the fundamental activities to be carried out in a certain way. They are not part of the fundamental reason for the existence of the product, but are necessary to make the product work in the desired way.
Non-functional requirements do not alter the functionality of the product. That is, the functional requirements remain the same regardless of the properties assigned to them. Non-functional requirements add functionality to the product: some pressure is needed to make a product easy to use or secure or interactive. However, the reason why this functionality is part of the product is to give it the desired characteristics. So you might think about functional requirements like those that do the job, and non-functional requirements like those that give character to the work.
Non-functional requirements constitute a significant part of the specification. They are important because the customer and the user can judge the product in its non-functional characteristics. As long as the product meets the required amount of functionality, non-functional properties - the useful, convenient, attractive and safe it is - can be the difference between an accepted and wanted product, and an unused product.