Non-Functional Requirement

The requirement says: “Build a payment API.” That tells you what it does. It doesn't tell you whether it must handle: 10 or 100,000 requests/sec 90% or 99.99% availability 1 second or 50 ms latency Those differences radically change architecture.

Specify required characteristics such as performance, availability, security and scalability. Functional requirements describe what the system does, such as accepting card payments. Quality attributes name the qualities that matter; non-functional requirements set specific expectations for them, such as completing 95% of payment requests within two seconds.