Rethinking the Notion of Non-Functional Requirements

Requirements standards and textbooks typically classify require- ments into functional requirements on the one hand and attributes or non-func- tional requirements on the other hand. In this classification, requirements given in terms of required operations and/or data are considered to be functional, while performance requirements and quality requirements (such as require- ments about security, reliability, maintainability, etc.) are classified as non- functional.
In this paper, we present arguments why this notion of non-functional require- ments is flawed and present a new classification of requirements which is based on four facets: kind (e.g. function, performance, or constraint), representation (e.g. operational, quantitative or qualitative), satisfaction (hard or soft), and role (e.g. prescriptive or assumptive). We define the facets, discuss typical combi- nations of facets and argue why such a faceted classification of requirements is better than the traditional notion of functional and non-functional requirements.