Here is what I think happens... Archived Apps and books are removed from the Nook so storage space is made available for new Apps or books. Barnes & Noble maintains servers that hold all the purchased and free Apps and books each account obtains. So this is why you can unarchive and item. When you unarchive an item it is simply downloaded again because the server knows you already purchased or selected the item. When you delete an App or book it is removed from both the Nook and the B&N servers so if you want to install that item again you will have to purchase or select it again.