If a user on an e-commerce site is in the middle of a purchase process when they leave to another part of the site, or perhaps leave the site completely, what information should be retained if they return to the checkout process? Should it keep their payment information? Should it ask for some sort of confirmation like "what are the last four digits of your payment card?" in order to retrieve that? Should it completely forget everything and just go back to a full cart as though no checkout attempt happened?