Functional are designed to do its certain function within a certain task. On the other end of the spectrum, non-functional are things that rather doesn't fit the certain task that its given; or it used to be functioning, though it doesn't anymore caused by use, age, development, etc. You can determine if "something" is still functioning whether it still performs the task given into it; and if the result of that performance from the functioning "something" was still excellent it the way it should be, we can still consider that "something" functional. And same goes with non-functional, but with a little wishy-washy within the premise of functional, we can safely assume that "something" is non-functional if it wasn't doing its given task accordingly to the result that we want. Thus, we can say that something still functions if it still does what it supposed to do, whereas non-functioning can be a alternative if the thing doesn't perform what it suppose to perform.
