This attribute is used where a persistent, unique user identifier is required that is explicitly consistent across different services. It often corresponds to the user's local single sign-on (SSO) name. It is especially useful where the identity has to be communicated out of band, perhaps by phone or email, or where access control lists are used. To support these uses there is an argument for constructing eduPersonPrincipleName (ePPN) from values with which users are already familiar.
UK Federation guidelines lead to a ePPN for someone at Cambridge having the form fjc55@cam.ac.uk. While textually identical to an @cam mail address, it is important to understand that this is neither an email address nor a Kerberos identity. This is likely to lead to user confusion.