You might want to ask CzechOut directly. They are the user that added that paragraph. As far as I am aware, the particular example they provided is not actually possible. However, they might know something I don't.
That help file has a strangely really bad example. I suspect it is because the wording is by a non-native English speaker. My guess is the example was trying to use the case where you put a page that uses a template in a different category based on an #if, but the wording uses "print" improperly.
I am pretty sure that isn't how the NAMESPACE magic word works. But yes, if you are specifically interested in distinguishing between file pages and articles, you can do that using the namespace instead. However, I still don't think you can distinguish between which categories a page belongs to.
Fandyllic, I hadn't thought of that. That could definitely be the case.