According to my observation requirements engineering in general doesn’t get the appropriate attention. If you consider software and/or hardware projects which failed, you can see that often a missing or incorrect requirements engineering is one of the major causes. If we go further and consider the types of possible requirements, then you can make a classification into functional and non-functional requirements as a first step. In general, the number of functional requirements, will be significantly higher than the number of non-functional requirements.
Usually, functional requirements are much more homogeneous than non-functional requirements. As the name implies, the functional requirements describe the function of the system to be developed. The validation of the correct implementation can usually determine consistently over a test. After a first version of architecture for the system is developed, a quite good decomposition for functional requirements can be performed. Read more