Functional versus nonfunctional requirements and project constraints
Key Takeaway
Behavior points to a functional requirement. A general or technical characteristic points to a nonfunctional requirement. A factor that limits the options points to a constraint.
Requirements are the needs a customer wants a solution to meet. They can be functional or nonfunctional. Constraints are the factors that narrow which solutions are possible. Requirements matter because they tell the people who build and test the solution what problem to solve and what to test. Requirements also give managers an easy way to track how the project is progressing.
What counts as a requirement
A requirement is something a stakeholder needs. To count as a requirement, a need must be quantifiable and verifiable. A request to make something prettier is not a requirement, because there is no way to measure it.