This IKERadiusAcctUserType enumerates which user-identifying field (xauthUserId, subjectAltName, or distinguishedName) to place in the RADIUS Accounting-Request User-Name attribute, here as priority 1. An admin orders these priorities so accounting records carry the most meaningful available identity for a dialup user, falling through to the next-priority field when the preferred one isn't present for that session. For example, an admin whose dialup users authenticate mostly via XAuth login rather than certificates would set priority 1 to xauthUserId so accounting logs reliably record the human-readable login ID.