An essential property of a software requirement is that it
should be possible to validate that the finished product
satisfies it. Requirements which cannot be validated are
really just “wishes.” An important task is therefore
planning how to verify each requirement. In most cases,
designing acceptance tests does this.
Identifying and designing acceptance tests may be
difficult for non-functional requirements (see topic 1.3
Functional and Non-functional Requirements). To be
validated, they must first be analyzed to the point where
they can be expressed quantitatively.
Additional information can be found in the Software
Testing KA, sub-topic 2.2.4 Conformance testing.