Yes, I agree that 'F' implies buy-on... I saw that on the PDF file at a flight that I am going to take and called the Cus. Rep. at reservation and asked them if any meal is offered on that flight (SFO-IAD), the Rep. told me that no, but I could buy them... (but on the other hand, I personally hate the food they sell... I was on a SEA-ORD flight and the food they sell was awful. It was just some beef jerk and stuff like that for a $5 charge... Luckily, I brought a salad at BK before departure... )

I think meal are offered in First class for any flight taking longer than 3 hours. I might be wrong. The best way to make sure is to call them...

And I remember a couple of years ago, they used to offer meal on transcon flight in E cabin (EWR-SFO)... I remember particularly one time I got shrimp salad or something... Can't believe that UA cuts it