Key points
- System requirements definition takes the business requirements and concretely defines "what should the system do?"
- It's organized into "functional requirements" (what features are needed) and "non-functional requirements" (performance, availability, and other conditions beyond features).
- The requirements settled here become the input for the basic design that follows.
Where it fits within upstream process
System requirements definition happens right after business requirements definition. Taking the goals and problems clarified on the business side, it defines exactly what the system needs to do.
📝Business requirements definition
→
⚙️System requirements definition
→
🖥️Basic design
How it differs from business requirements
Business requirements describe "what we want to achieve" (the goal), while system requirements define "what the system will do to get there" (the means). Even for the same problem, the level of detail changes as you move through these stages.
Teacher Pochi's hintIn renovation terms, a business requirement like "we want more storage" (a lifestyle wish) gets refined down to floor-plan-and-spec detail in system requirements definition — "install 2 tatami-mats' worth of wall-mounted storage" or "use sliding closet doors."
What are functional requirements?
Functional requirements describe exactly what the system does — the features themselves.
For example, "users can register an order," "stock levels display in real time," and "a confirmation email is sent after an order completes" are all functional requirements.
What are non-functional requirements?
Non-functional requirements are the quality conditions the system must satisfy beyond its features — things like performance (response speed), availability (staying up), security, and scalability.
PerformanceReturn search results within 1 second
AvailabilityMaintain 99.9% or higher uptime per year
SecurityStore personal data encrypted
Teacher Pochi's hintThink of a house: functional requirements are "what each room can be used for" (kitchen, bedroom, and so on), while non-functional requirements are things like earthquake resistance, insulation, and security — conditions that are hard to see but shape the quality of everyday life.
How functional and non-functional requirements relate
In practice, requirements definition organizes these two categories separately and checks each for gaps.
⚙️ Functional requirements (what it does)
Order registration
Stock display
Notification sending
🛡️ Non-functional requirements (quality conditions)
Performance
Availability
Security
Scalability
Practical pitfalls to watch for
Non-functional requirements get sidelinedFocus too much on functional requirements and performance/security get shortchanged, leading to major rework right before release.
Use a shared framework to align on levelsA published framework — such as Japan's IPA "non-functional requirements grade" — helps ensure nothing gets missed when agreeing on target levels with stakeholders.
Prioritize requirementsTrying to max out every requirement inflates budget and timeline, so agree upfront on what's a MUST versus a nice-to-have (WANT).
Summary
System requirements definition takes the business requirements and turns "what should the system do?" into something concrete, across both functional and non-functional requirements. What's settled here becomes the input for the basic design that follows.
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