Many times when I've used Outlook I have used the follow-up flag to remind me about the item that I'm looking at. Sometimes I use it just so that I can see it in a pile of things within my mailbox, other times I use the Outlook search folder and look at the things that I've marked for follow-up and then I process them.
But what happens when the item gets archived?
Well, the best way to consider this situation is that archiving effectively creates a freeze-frame of the item. If the item was archived with a flag set then the archived item will always have a flag set. If the item was archived without the flag set, then yes, it can be set later, but if you restore the original item, it won't have a flag.
It's fairly logical, but it does sometimes lead to confusion when a user clears the flag on an archived item, then at some point later then double click on the archived mail to view, and they see the reminder again (even though they've cleared it)
As I said the way to remember things is that the archived item is a snapshot or freeze-frame of the item AT THAT PARTICULAR point.
If you use flags, how do you find using them when archived messages are involved? Let me know in the comments.