Functional Requirements

Definition

Functional requirements (FRs) describe what the system should do — the specific functions, features, tasks, or services the software must perform in response to given inputs, including how it should behave in particular situations.

Characteristics

• Describe behavior and functionality (actions, calculations, data manipulation)
• Usually expressed as: "The system shall [do X] when [condition Y]"
• Directly testable — pass/fail based on observed behavior
• Derived from user needs, business rules, and use cases

Examples

• "The system shall allow users to register with an email and password"
• "The system shall calculate the total bill including tax and discounts"
• "The system shall send an email notification when an order is shipped"
• "The system shall allow an admin to delete a user account"

How Functional Requirements Are Captured

• Use Cases — describe interactions between actors and the system
• User Stories (Agile) — "As a [user], I want [goal] so that [benefit]"
• Process/Data Flow Diagrams
• Decision Tables/Trees for complex conditional logic

Categories of Functional Requirements

1. Business rules — logic/policies the system enforces
2. Transaction corrections/adjustments — undo, cancel, edit operations
3. Administrative functions — user management, configuration
4. Authentication/authorization — login, access control
5. External interface requirements — how the system interacts with other systems
6. Certification requirements — compliance-related functional behavior

Functional vs. Non-Functional (quick contrast)

• Functional = "What" the system does
• Non-Functional = "How well" it does it (performance, security, usability)

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Exam tip: Always give 2–3 concrete examples in "shall" statement format when asked to define FRs — examiners specifically look for correctly formatted requirement statements.