It indents the text, like in a quoted reply in your email.
I'm sure you've seen them, when someone responds and the original email has a > mark before each sentence, then it grows with each subsequent response >>, then >>> and so on. That way it designates the old email from the new.
To me means it will put a sold line down the left side of the email to indicate the previous (or quoted/replied to) content. So you don't get it confused when typing your reply.
Edit. I was typing my response as u types the above one in which you are correct.