Functional vs Extra Functional Requirements

Today's industrial practice largely separates the design for functional and extra-functional requirements. While the functional design transforms functional requirements into a logical model that consists of objects and idealized interactions, the extra-functional design (the "architecture") transforms extra-functional requirements into an architectural model that consists of components and possibly frameworks. Merging both design paths yields the final system.

Extra-functional requirements (also known as nonfunctional requirements) are constraints regarding quality (e.g. usability, performance, security, maintainability) and economics (e.g. time, cost) of process as well as product components.