What this resource covers
Functional requirements describe what users and systems must be able to do. Nonfunctional requirements describe the conditions and quality expectations under which those functions must operate. Both affect architecture, effort, testing, hosting, and risk.
- Describe user actions, system responses, calculations, validations, approvals, notifications, imports, exports, and integrations
- Define access control, auditability, privacy, performance, availability, capacity, compatibility, usability, backup, and recovery needs
- Express quality requirements in testable language where the organization can provide meaningful targets
- Classify requirements by importance and release phase
- Trace requirements to design decisions and test scenarios
Information to prepare
Prepare user goals, business rules, transactions, reports, interfaces, expected volume, availability, response-time needs, security roles, retention, recovery, accessibility, and support expectations.
- Approved workflows and user roles
- Expected data volumes, user concurrency, and operating hours
- Security and privacy policies
- Hosting, browser, device, integration, backup, and recovery expectations
- Business-critical tasks and acceptable service interruption supplied by stakeholders
Expected planning outputs
The result should separate functional behavior from measurable quality attributes, link both to owners and acceptance methods, and expose conflicts such as performance, security, cost, or usability tradeoffs.
- Functional requirement catalog
- Nonfunctional requirement catalog
- Acceptance criteria and quality targets
- Requirement priority and release mapping
- Traceability between requirements, design, and testing
Practical example
How it can be applied
A functional requirement may state that billing staff can create an invoice from completed trips. Related nonfunctional requirements may define who can perform the action, what audit record is retained, acceptable response behavior, and how invoice data is backed up.
Common mistakes to avoid
- Documenting features while ignoring security and recovery
- Writing quality words without measurable or reviewable criteria
- Copying targets from another system without business evidence
- Treating every requirement as equally urgent
- Leaving nonfunctional testing until the end
FAQ
Common questions
Is security a functional or nonfunctional requirement?
Security often includes both. Login or permission checks are system behavior, while broader confidentiality, audit, and protection expectations are quality constraints.
Who defines performance expectations?
Business and technical stakeholders should define realistic operating needs using expected users, data, workflows, and service importance.
Why do nonfunctional requirements affect cost?
They can change architecture, infrastructure, engineering effort, testing depth, monitoring, and operational support.