Every user in SCCM that is being discovered by SCCM 2012 from Active Directory has a Unique SID assigned to it . Config Manager recognizes every user in SCCM 2012 using this SID which eventually is seen  a user in the Console.

When you double click a user in Console you will find that it has a unique SID attached to it :

Now see the below snap Shot you will find that in the rebootcoordinator.log , SCCM identifies users using this SID :