Functional vs Non-Functional Requirements

Functional Requirements

Describe WHAT the system must do — the specific functions, features, services, and behaviours the system must perform. They define inputs, processing, and outputs.

Examples:

• "The system shall allow a user to log in with username and password."
• "The system shall calculate and display the total bill including 18% GST."
• "The system shall send an e-mail notification when an order is shipped."
• "The ATM shall dispense cash only after verifying the PIN."

Non-Functional Requirements

Describe HOW well / under what constraints the system must perform — quality attributes and constraints on the system (not specific behaviours).

Categories & examples:

• Performance: "The system shall respond to a query in under 2 seconds."
• Reliability: "The system shall have 99.9% availability."
• Security: "Only authorized users shall access financial data; passwords stored encrypted."
• Usability: "A new user shall be able to complete registration without help."
• Maintainability/Portability: "The system shall run on Windows, Linux, and macOS."
• Constraints: "Must run on 4 GB RAM; must comply with GDPR."

Comparison table

Aspect	Functional Requirements	Non-Functional Requirements
Defines	What the system does	How well / constraints
Nature	Behaviour/features	Quality attributes
Verifiable by	Functional test cases	Performance/security tests, review
Example	"Withdraw cash", "send email"	"Response < 2 s", "99.9% uptime"
Failure means	Feature missing/wrong	System slow, insecure, unreliable
Priority	Core capability	Often critical for acceptance
Where captured	SRS "specific requirements" section	SRS "non-functional requirements" section

Why the distinction matters

• Ensures both capabilities (functional) and quality (non-functional) are specified and testable.
• Non-functional requirements (especially performance, security, reliability) are as important as functional ones — a system that does the right thing too slowly or inseccurely is not acceptable.

Summary: functional requirements answer what, non-functional answer how well — both are essential in the SRS and both must be verifiable.