Learn why the user authentication process is a classic example of a functional requirement.

Let me explain a core idea that shows up in every good software project: a system has functions it carries out, and it has qualities that affect how well it does those jobs. In the world of requirements, those two things are called functional requirements and non-functional requirements. They live on the same page, but they describe different kinds of expectations. For anyone studying IREB Foundation Level concepts, this distinction is a trusty compass.

Functional vs non-functional: what’s the difference, in plain terms?

• Functional requirements describe what the system should do. They specify actions, behaviors, and interactions from a user’s point of view. Think of them as the tasks the system performs.

• Non-functional requirements describe how the system should be while performing those tasks. They cover things like speed, availability, reliability, and security. They’re about quality, not specific actions.

Let’s anchor this with a concrete, simple example that often comes up in discussions about requirements: user authentication.

A real-world example of a functional requirement

A is the winner in many quizzes for a very good reason: it names a specific function. The user authentication process is something the system must do, and it’s described in terms of user behavior.

• How a user interacts with the system to verify identity: entering a username and password, the system checking those credentials, and deciding what happens next.

• The details that matter for the user: what happens if the credentials are correct, what happens if they’re wrong, and what flows happen when a user logs in or out.

So, the precise functional requirement might look like this when it’s written out in a real project: “The system shall authenticate a user by validating a provided username and password, deny access for invalid credentials, and provide a clear error message.” That sentence nails down the behavior from a user’s perspective. It tells developers what to implement and testers what to verify. It’s concrete, testable, and directly tied to an actual action a person performs.

Non-functional examples: why B, C, and D aren’t the same thing

Now, let’s unpack why the other options—uptime, response time, and data processing speed—are non-functional. They describe how well the system performs its duties, not the duties themselves.

• System uptime: This is about availability. It answers questions like “How often is the system up and reachable?” It’s a reliability and maintainability concern, not a specific action users expect the system to perform.

• Response time: This is about the speed of interaction. It asks, “How quickly does the system react to a user action?” It’s a performance metric, not a defined function.

• Data processing speed: Similar idea, but focused on throughput and efficiency. It’s about how fast data can be handled, scaled, or transformed under certain conditions.

All of these influence the user experience, and they’re vital for design decisions and testing plans, but they don’t specify a particular function the system must perform. In other words, they describe the how, not the what.

Why this distinction matters in practice

Understanding the split between functional and non-functional requirements isn’t just about grammar in a requirements document. It shapes:

• Scope and boundaries: Functional requirements carve out the exact behaviors the system must deliver. Non-functional ones set the quality envelope—how well those behaviors must hold up under real-world use.

• Acceptance criteria: When you test, you need to know what to check. If a login function exists, you test successful login with valid credentials and failure with invalid ones. If you’re testing uptime, you verify the system is reachable most of the time. Different tests, different goals.

• Design decisions: A functional requirement might push you toward a particular UI flow or API, while non-functional requirements push the team toward resilience, caching strategies, or security controls.

• Change management: If user needs shift, it’s often the functional side that adapts. If performance expectations change, the non-functional side—like response times and uptime—gets tuned.

How to write functional requirements that actually guide the team

Great functional requirements are crisp, verifiable, and user-centered. Here are some practical tips you can use, with quick examples:

• Be action-oriented: The requirement should state an observable action. Example: “The system shall authenticate a user using a valid username and password.”

• Identify the user role (the actor): “A registered user shall be able to log in.”

• Define the expected outcome: “The user shall gain access to their own dashboard after successful authentication.”

• Include edge cases and error handling: “If the credentials are invalid, the system shall present a clear error message and allow a retry.”

• Tie to business goals: “Only authorized users shall access protected resources.”

Common normalizations you’ll often see

• Positive framing: “shall” or “will” to show a mandatory behavior.

• Clear boundaries: avoid vague phrases like “the system should be able to,” which leaves room for interpretation.

• Testability: always keep a way to verify the requirement. If you can’t test it, you probably need to rewrite it.

A quick compare: good vs not-so-good functional statements

• Good: The system shall authenticate a user by validating a unique username and a password, and shall create a session upon successful login.

• Not-so-good: The system should handle login smoothly.

The first is precise and testable; the second is vague and leaves testing to interpretation.

A few notes on non-functional considerations (soft anchors you’ll meet soon)

• Availability and uptime: You’ll often see targets like “99.9% up time,” but remember this is a performance expectation rather than a function.

• Response time and throughput: It’s common to specify maximum response times for key actions (e.g., login, search) or minimum throughput for batch jobs.

• Security and privacy: Some security requirements are non-functional, but others can be functional if they require specific behaviors (like MFA prompts or a password change flow). It’s a gray area that savvy teams track carefully.

• Usability and accessibility: These relate to how usable or accessible the features are, still considered non-functional but highly important for user satisfaction.

A friendly nudge about realism and completeness

Functional requirements shine when they’re tied to real user workflows. If you map a persona—say, “a returning customer who forgot their password”—you can craft a sequence of functional steps: request a password reset, receive a link, set a new password, and sign in. This approach keeps the requirements grounded in actual behavior, not in abstract ideals.

A tiny digression that still matters

While we’re at it, consider the human side of a login flow. In the real world, people forget passwords, get locked out, or switch devices. A thoughtful functional requirement learns from that reality: it might include a safe, user-friendly password reset process, or a simple way to log out from all devices. Those touches aren’t glamorous, but they keep the system humane and trustworthy. And yes, they often show up in the functional section because they describe concrete user actions and responses.

Bringing it all together: what to remember

• Functional requirements specify what the system does—the exact actions, interactions, and behaviors a user can expect.

• Non-functional requirements describe how well the system does those things—the quality, performance, and reliability aspects that color the user experience.

• The login example is a textbook case of a functional requirement: it defines a concrete interaction between the user and the system.

• B, C, and D aren’t incorrect in their own right; they simply belong to the non-functional category—helping you understand the system’s character rather than its explicit actions.

If you’re exploring these ideas, keep a simple mental model handy: imagine walking a user through a website or app. What are they asked to do? What happens next? Those “do” steps are your functional requirements. How fast, secure, available, or accessible the journey feels—that’s the non-functional stuff shaping the ride.

A few practical steps you can apply right away

• Start with user stories when drafting functional requirements. “As a user, I want to log in with a username and password so I can access my dashboard.”

• Add acceptance criteria that are testable. For the login example, acceptance includes successful login with valid credentials, error message for invalid credentials, and a session created on success.

• Keep non-functional needs explicit but separate. For a login flow, specify acceptable response times and security constraints, but keep them apart from the step-by-step login actions.

• Review with stakeholders from both sides—business, design, and engineering. The goal is consensus on what must happen and how well it must perform.

In the end, this isn’t just a checkbox exercise. It’s about building software that people can rely on—one that behaves predictably, feels trustworthy, and serves real needs. When you frame requirements with that mindset, you’ll notice how the pieces click into place sooner rather than later.

If you’re curious to dive deeper, look for real-world examples in your next project briefs or product backlog: see how they separate the “what” from the “how well.” You’ll likely spot lots of functional statements in the mix, along with non-functional targets that guard the experience. And that balance—that’s where the magic happens, keeping teams aligned while still moving fast.

Key takeaway at a glance

• Functional requirement: a specific action a system must perform (example: user authentication).

• Non-functional requirement: a quality the system must exhibit while performing (examples: uptime, response time, processing speed).

• Writing tip: use clear, testable language that ties directly to user behavior; separate the “what” from the “how well.”

So next time you see a statement about how a system should behave, ask: does this describe a concrete action the user experiences, or is it a measure of performance and quality? Keep the distinction in mind, and you’ll navigate requirements with confidence, clarity, and a touch of practical wisdom.