What does "in mode u" mean in yum output?

Issue

While running "yum update" below output is observed:

...
Adding Package nss-tools-3.13.5-8.el5.x86_64 in mode u
Adding Package yum-fastestmirror-1.1.16-21.el5.noarch in mode u
Adding Package libgcc-4.1.2-54.el5.x86_64 in mode u
Adding Package gnome-system-monitor-2.16.0-4.el5.x86_64 in mode u
...