AU just refers to a class of service and ato (airport) entry that can be used to upgrade a passenger. I am assuming that you were not booked in Y, and the long and short of it is, that you got a nice agent who didn't care about the capacity controlled booking class BS, etc. F class is one of CO's capacity controlled first class booking codes. The airport agents have computer entries that can pretty much over-ride any rule, parameter, procedure, term/condition, tariff, obstacle, impediment, or other such thing that could make your travels more difficult and less rewarding. If an airport agent ever tells you first is full say "can you just AU the segment," and if they like you they may just AU it and you will be in first regardless of whether CO yield management wanted you in that seat at that time.