Definition:

extent to which behaviours or outcomes, or both, of a product meet (satisfy) specified quality requirements when used.

ISO/IEC 25019:2023, §3.2.1.3

Suitability is a quality-in-use characteristic. It asks whether the outcomes of using a product satisfy the quality requirements specified for it — whichever qualities those requirements happen to concern. A waterproof device is thereby suitable for underwater use, whatever functions it offers.

Functional suitability asks the narrower question of whether a product provides the functions that meet stated and implied needs. The two are defined in different SQuaRE standards — suitability in the quality-in-use model ISO/IEC 25019, functional suitability in the product quality model ISO/IEC 25010 — so they are not synonyms.

The #suitable dimension takes the term in this broader sense.