I think that when we say we "use a function" we really mean we pay attention to the information that is described by the "function" label.
For example: I more often pay attention to the information set that is labeled as Ne. At times however, I need to pay more attention to a different information set. When I attempt to describe those times when I'm using a particular information set, saying that I'm using Fi or Se or whatever is a shortcut in my communication efforts.

Another example is "How do I function?" "I function by paying more attention to Ne and Fi information than I do other information." or "I don't function as well when I am forced to pay attention to Ti information."

Please take into consideration though that I have issues with Model A and it's set up. So what I'm referring to when I refer to "function" may differ from what someone else is referring to when they say "function".