This is, essentially, how the system will work under the hood. A lot of the key requirements will be buried in this section. An example of a functional requirement for guitar hero would be the scoring mechanism or even the combo mechanism. You can think of them as the game mechanics.

Considering a more sedate, if not a bit duller, example we can think of Microsoft word. Some of the functional requirements would be things like auto correction, word count, spell check and many of the other features. All of these features, quite rightly, have a user interface element. What we are interested here is what they do when the user clicks on them. The requirements should not dictate what the features should look like. In fact it is very common that features described will have no possible user interface feature. What the functional requirements must focus on is simply what do they need to achieve.