What is the difference between a Functional Requirement and a Quality Requirement (Non-Functional Requirement)?

Functional = what the system does (a feature); Quality/non-functional = how well it does it (performance, availability, security…).

A Functional Requirement defines behavior/functionality the system shall provide. A Quality Requirement (Non-Functional) addresses quality concerns not covered by functional requirements, such as performance, availability, and security.

Functional Requirements:

• Concern the behavior/function provided by the system
• Usually divided into:
  • Functional requirements (features)
  • Behavioural requirements (how it behaves)
  • Data requirements (what data it handles)

Quality Requirements (Non-Functional):

• Pertain to quality concerns beyond functionality
• Typical areas:
  • Performance
  • Availability
  • Dependability
  • Scalability
  • Portability
  • Security

Tip: Functional = "What it does", Non-Functional = "How well it does it"