That's now insULT fares, as a non-upgradable T class has been added as well.
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by bikenski:
Biggest reason to not want this to go through is DL's attitude on domestic upgrades, and the possibility that NW and CO would follow suit. They will NOT let you upgrade on the lowest fare classes (L and U, more commonly referred to as LUser fares over on the DL board.)
We currently enjoy an excellent domestic upgrade program, and I would hate to see the benefits stripped away because of DL.</font>