Sometimes, we don’t want full anonymity. You may want to order flower bulbs but not be placed on numerous mailing lists for gardening supplies. But you also want to be able to place similar orders again, asking for the same color tulips as before. This situation calls for pseudonyms, unique identifiers that can be used to link records in a server’s database but that cannot be used to trace back to a real identity.
Multiple identities can also be convenient; for example, you may have a professional email account and a social one. Similarly, disposable identities (that you use for a while and then stop using) can be convenient. When you sign up for something and you know your email address will subsequently be sold many times, you might get a new email address to use only until the unsolicited email becomes oppressive. Seigneur and Jensen [SEI03] discuss the use of email aliases to maintain privacy. These uses, called pseudonymity, protect our privacy because we do not have to divulge what we consider sensitive data.