It is the carrier profile for the phone. If I am correct in my information, which I may not be 100%, it is a configuration file (or a script) that appears to install AT&T links on the phone, as well as tweaks the settings a little.
Appears to possibly hide some options and enable some others that are already accessible on unlocked phones. Could also include tower data for possibly better 3G reception (this data is normally updated with SIM card upgrades).