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I have been doing some research and realized that I missed the fact that in ISO 9126 and ISO 25010, all characteristics are quality characteristics and represent (according to various papers) just non-functional ones. It surprised me as I thought "Functionality" is clearly functional but now I can see that they probably mean "evaluation of functionality from the non-functional view, i.e. how is the system doing these things".

Then I found an interesting thing in an ISTQB book. They talk about functional tests and reference ISO 9126 Functionality. But in the non-functional testing section, they enumerate non-functional requirements according to this ISO and do state Functionality, referencing the section in Functional testing page!

ISO 25010 is said to present 8 non-functional categories, including Functional suitability. That does sound like a non functional one but again it is said it is just renamed Functionality from ISO 9126. And yet there are books saying that "Functional suitability" from ISO 25010 is functional and should be tested functionally (well, it does make sense more than saying it is non-functional)...

Most of the QA books typically lists ISO 9126 Functionality under Functional testing so I cannot understand how this can be listed as non-functional...

Similarly, I found for the ISO 9126 sources claiming that its Functionality is quality requirement, therefore non-functional requirement, while the other sources state that non-functional are those except for this one.

Does anyone know this ISO standard well enough to explain the problem?

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Look at the quality model listed here. Every one of those qualities is non-functional.

Functionality that fulfills the business's needs is determined by your functional software requirements. Each functional requirement should be accompanied by an acceptance test, and it is that acceptance test (and not some ISO standards document) that dictates whether or not that functional requirements is fulfilled.

Hypothetically, an ISO document might address the way the requirement and its acceptance test are written and executed, but not the nature of the functional requirements themselves. Functional requirements are wholly dependent on the needs of the stakeholders.

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    I'm not an expert on the ISO standards referenced, but I think this is correct. They are trying to say that it doesn't matter if your software works, the functionality or functional suitability (functional suitability is better, IMO), is the non-functional site of functionality. Is your working software suitable for the customer/user's needs? Does your software produce accurate results? Does it work within the context of a larger system? Is it secure and meet regulatory standards? These are all non-functional questions, since you can make working software that doesn't meet them. Commented Jun 10, 2015 at 14:29
  • @Robert Harvey: Maybe it is my English but I am not sure what you are saying. You say these qualitiies are non-functional (including Functionality) but I cannot see how? Commented Jun 10, 2015 at 14:35
  • I'm saying that ISO 9126 has nothing to say about functional requirements. Commented Jun 10, 2015 at 14:37
  • Thanks. I would think the same..however, I read the external metrics which mentioned measurements of individual sub-characteristics and - as adopted by some ISTQB books - they say e.g. Correctness - verify expected or implied output as stated in functional requirements. IMO this has nothinh to do with NFR! Im just confused... Commented Jun 10, 2015 at 14:40
  • Yes, that's essentially what the last paragraph of my answer says. It may sound counter-intuitive, but "correctness" (i.e. the ability of a program to pass its acceptance tests) is a non-functional requirement. Commented Jun 10, 2015 at 14:47

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