Hmmm, guess it is just what it says, for quick exchanges back and forth. In the subject line it automatically puts in the name of the person who posted right before you.

So, you don't use this if you are responding to somebody else farther up the thread. Right? In that case you would click on "Reply" in the specific post and user you are answering.

I'm still not sure what it is or does. Somebody else please figure it out for me.