These functions meet the defined requirements. In product design , functionality is one of the most important aspects. In the software requirement , functionality - in addition to changeability / maintainability , usability , efficiency , transferability and reliability - is an essential quality feature .
Sub-goals of the functionality are according to ISO / IEC 9126 (successor ISO 25000):
- Appropriateness: Suitability of functions for specified tasks, for example task-oriented composition of functions from sub-functions.
- Security : Ability to prevent unauthorized access, both accidental and deliberate, to programs and data.
- Interoperability : Ability to interact with given systems.
- Conformity : Ability of the software product to comply with standards, conventions or legal provisions and similar regulations with regard to functionality.
- Compliance: Features of software that cause the software to comply with application-specific standards or agreements or legal provisions and similar regulations.
- Correctness : Delivering the correct or agreed results or effects, for example the required accuracy of calculated values.
- Ulrich Wohlgemuth: Machine Design. Designing industrial products successfully , Diplomica Verlag , 2016 ISBN 978-3-95934-966-6 p. 59 
- Christof Ebert: Systematic Requirements Engineering: Determine, specify, analyze and manage requirements . 5th, revised. Edpunkt , Heidelberg 2014, ISBN 3-86490-139-1 , p. 86 .
- Helmut Balzert: Textbook of software technology. 1, basic concepts and requirements engineering . 3. Edition. Spectrum, Akad. Verl, Heidelberg 2009, ISBN 3-8274-1705-8 .