What is the difference between functional and non-functional requirements?

What is the difference between functional and non-functional requirements?

Functional requirements describe what a system should do. Non-functional requirements describe qualities or constraints that affect how the system behaves and operates. Both influence design, implementation, and testing; a product can perform its main actions while still failing to provide an acceptable experience.

Describe actions with functional requirements

In a wedding-management app, functional requirements might say that an organizer can create an event, assign a task to a collaborator, and view guest responses for each event.

These requirements specify actors, inputs, rules, and outcomes. A useful description also covers exceptions: an event with an invalid date is rejected, or a collaborator cannot assign work outside the weddings they can access.

Acceptance criteria make each behavior observable.

Make quality expectations measurable

Non-functional requirements cover performance, availability, accessibility, maintainability, privacy, or operational recovery. “The dashboard should be fast” is too vague to evaluate.

A clearer requirement identifies the action, target, and conditions. For example, the team might propose a dashboard response-time target under a defined load and representative dataset. That target must be realistic and agreed rather than invented by an AI assistant.

Accessibility expectations should also identify relevant interaction needs and verification methods, such as keyboard access for core workflows.

Evaluate the requirements together

The distinction is a useful organizing tool, not an absolute classification rule. Security can involve both functional behavior, such as revoking an invitation, and broader constraints, such as enforcing access boundaries consistently.

Ask AI to separate vague statements into specific expectations and identify how each could be checked. Then review the resulting requirements against product priorities and practical constraints. Both categories belong in planning because neither can be reliably added through visual polish alone.