Advanced Authentication shows a name from the first, non-empty specified field for an entered user name.
For AD, the default attributes are sAMAccountName and userPrincipalName. For other repositories, cn is the default attribute.
NOTE:Points to consider about the otherMailbox attribute:
Advanced Authentication 6.4 Service Pack 2 Patch 1 includes the otherMailbox attribute as default attribute in User Name attributes.
When you upgrade to Advanced Authentication 6.4 Service Pack 3 then the otherMailbox attribute is supported as default attribute in the existing repository. However, for a new repository, the otherMailbox attribute is not supported as default to improve search and performance.