A personal archive is for individual users. Basically, each user has two mailboxes. One is the primary mailbox which has all the current email and the other is a mailbox which will has all of the older emails — which is called a personal archive. The drawback is that any user can delete the emails from a personal archive just as they delete the emails from their primary mailbox. So, if you are trying to preserve the emails then you will have to place legal hold on all the mailboxes.