The functions that a system, subsystem or component are to perform may be described in work products such as a requirements specification, use cases, or a functional specification, or they may be undocumented. The functions are “what” the system does.
Functional tests are based on functions and features (described in documents or understood by the testers) and their interoperability with specific systems, and may be performed at all test levels (e.g. tests for components may be based on a component specification).
Specification-based techniques may be used to derive test conditions and test cases from the functionality of the software or system. (See Chapter 4.) Functional testing considers the external behaviour of the software (black-box testing).
A type of functional testing, security testing, investigates the functions (e.g. a firewall) relating to detection of threats, such as viruses, from malicious outsiders. Another type of functional testing, interoperability testing, evaluates the