to clarify the usage of "Vos"

In Lesson 2.10, Vos is defined as "Your (formal or collective, plural noun)" Please don't tell me there is a formal noun?! Or does it mean "Your (formal) used with a collective or plural noun"? Also could you give me an example or two of a collective noun?

The french possessives in front of a noun to identiy which person owns it are guided by two things as I understand it,: the number of the person and the number of the things. Only when there are two or more of the person (whether 1st, 2nd, or 3rd) or the reference is a collection like "group" AND there are two or more of the things, AND the person is second person, only then do you use "vos". So if you're talking to Mary about her books you would say "votre livres", but if you're talking to Mary and Same about their jointly owned books, you would say "vos". Marie-Claire?