If the user name contains upn (user@domain.com) or SAM (domain\sam), NOT prepending the domain name or netbios name while sending the response to challenge. (TYPE 3 token).
If the username doesn't fall under category either UPN or SAM, prepending the domain name coming from (type 2 token) in the username while sending the response to server challenge.
Support for UPN
If the user name contains upn (user@domain.com) or SAM (domain\sam), NOT prepending the domain name or netbios name while sending the response to challenge. (TYPE 3 token).
If the username doesn't fall under category either UPN or SAM, prepending the domain name coming from (type 2 token) in the username while sending the response to server challenge.
upn (user@domain.com) -- \user@domain.com
SAM (domain\user) -- domain\user
username (user) -- DOMAIN\user.