Citing multiple references in one number
You are right about the function of this, but I think it's often not the best practice (although this is strictly my own opinion).  Personally, I avoid combining references entirely.  In my opinion, it can be used legitimately, when the papers combined into a single reference are by (mostly) the same authors and cover the continuous development of a single topic.  However, I mostly see it used by people who want to pour a large number of references into a single number, perhaps to save a little bit of space.