It is also my understanding that the hd is not unique to each user. However what would not delete if user "a" ran a wipe free space, what would still be visible for user "b"?
Basically I had two logins one a computer, and three other people had user logins. I ran ccleaner on both of my user logins, but only ran wipe free space for user "A". what still exists on user "B" login?