Understanding USMT 4.0 Behavior with UEL and UE
From the archive (2010). Written for the tools of the time, so some details may be out of date.
Great article by Ned here that discusses the use of /UE and /UEL with USMT 4.0
In addition to the article by Ned, Nick Moseley has a post he created on his experiences with excluding users using USMT 4.0
http://t3chn1ck.wordpress.com/2010/08/13/excluding-users-using-usmt-4-0-in-sccm-sp2/