We want to adopt ISO 25000 quality model, which explicitly states it does not deal with functional requirements (i.e. its Functional suitability is truly about assessing how the functions fit in the context etc.).
According to this and other sources, quality is a non-functional attribute, basically a sum of quality attributes.
But - what about the functional requirements? Are they not part of quality? Some state that "what" (functions) is not enough to perceive quality ("how" - fast, efficient, robust...), but on the other hand, if there are bugs in the functionality, isn't an indication of bad quality? Even though following the 25000 model, they are out of scope of its asssesment.
But I certainly cannot have a perfectly robust, efficient, fast software that just does not according to its functional specs..yet following this standard, it would be the case.
Personally, I would say that the overall quality must include both functional quality AND non-functional, yet I have never read anything like that.