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Among the requirement that we can collect from the application

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domain, we need to distinguish between two main types. And you've

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probably heard about these ones.

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Functional requirments and non-functional requiremnts. Functional

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requiremetns have to do with the functionality of the system, with

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what the system does with the computation. For example the elevator

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shall take people to the floor they select. That's a functional

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requirement, that has to do with the functionality of the system.

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Or for a very simple one, the system has to output

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the square root of the number past as

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an input. So these kind of requirements have in

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general well defined satisfaction criteria. So, for example, if

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for the latter one that we mentioned it is

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pretty clear how to check whether the output

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is actually the square root of the number passed

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in input. Non-functional requirements, conversely, refer to a system's

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non-functional properties, systems qualities.

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Such as security, accuracy, performance,

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cost. Or, you know, usability,

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adaptability, interoperability, reusability and so

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on. So, all these qualities the don't necessarily have to

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do with the functionality. And, unlike functional requirements, non functional

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requirements Do not always have clear satisfaction criteria. For example,

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if we say that the elevator must be fast, that's

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a non-functional requrement. Right? It has to do with the

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speed of the elevator, which is a quality of the

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elevator. But, it, it's not clear how such a requirement

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could be satisfied. How could we tell whether the elevator

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is fast or not. So, what we need to do

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in these cases Is that we need to refine these

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requirements so that they become verifiable. For the example that

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I just mentioned, for instance, we might say that the

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elevator must reach the requested floor in less than 30

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seconds from the moment when the floor button is pushed.

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This is still a non-functional requirment, but is a verifiable one.