I define a functional requirement as a requirement about a result or behavior that a system function must provide. In short, I describe what the system shall do. Therefore, I focus on required results, behaviors, or interactions. For example, I state that a customer entry form shall contain fields for a customer’s name and first name. In addition, I can specify limits such as up to 32 characters per field. Even though I may add detailed presentation rules, I still treat that as a functional requirement. However, I avoid confusing functional statements with other requirement types.
First, I ask what concern the requirement addresses. If the concern targets a required result, behavior, or interaction, then I mark it as functional. If the concern instead targets qualities like reliability, security, scalability, or performance, then I treat it as a quality requirement. For example, the need to handle enormous detector data volumes or to meet a particular processing speed concerns performance. Consequently, I classify those as quality requirements, because they address quality concerns that go beyond a single function’s direct behavior. Likewise, if an item restricts the solution space—such as mandating a specific technology or a legal constraint—I label it a constraint.
Next, I avoid relying solely on the common rule “what the system shall do versus how the system shall do it.” That rule often misleads. For instance, a very detailed form layout still answers what the system must provide. Thus, it remains a functional requirement. Conversely, performance demands often appear like functional needs when stakeholders describe them. Nevertheless, I treat them as quality requirements if they address non-behavioral concerns.
Moreover, I use three useful dimensions when I organize requirements. First, I consider the type of requirement: functional, quality, or constraint. Second, I evaluate the requirement’s dependence on a solution. For example, I distinguish goals from solution-bound process descriptions. Third, I check the level of detail or abstraction. For instance, I record high-level goals and detailed function descriptions separately. Therefore, I can map and manage requirements more clearly.
Furthermore, I aim for clarity. Therefore, I write functional requirements in short sentences. Also, I avoid ambiguous words. When possible, I include acceptance criteria. In addition, I keep solution-independent goals separate from detailed function statements. Thus, I reduce the risk of misclassification.
Finally, I note scope limits. I do not cover requirements for project management or development processes here. Instead, I focus on requirements that describe the planned system’s change or behavior. In practice, I recommend you review requirements with stakeholders. Then, you clarify whether each item addresses a result, a quality, or a constraint. Consequently, you improve classification and design decisions. In summary, I define functional requirements as statements about required system behavior or results. Therefore, I treat them as the core of what the system must do, while I separately manage quality concerns and constraints.