Originally Posted by qasr
So you can't upgrade it to F, duh.

No need to be rude.
OK, so it does not allow this fare on three class planes, so why have it? I just did a dummy one-way booking on UA 243 (three class 777 ORD-DEN at 545pm). Selecting Business class gave be a CUA fare of $1000. Selecting First gave me a FUA fare of $960.