Originally Posted by
xliioper
It can likely be used on C+ fares if the underlying coach class is high enough (all C+ fares have an underlying coach class). It's the first letter of the Fare Basis code for the fare which can be found in the "Fare Details:" line of the email receipt. This is how Mileage Upgrade Awards work on C+ fares. The 50K point Select Fare upgrades require a K or higher fare class. I wouldn't really call this "most" main cabin fares as it excludes LUTXV fares (you need a YBMHQK fare to apply it). The 25K point "Full Fare" upgrades require YBM fares to apply.
Interesting. And very helpful, thanks. So for example if I'm seeing this:
Fare Details: NYC DL SEA222.33VAVSA0C1 USD222.33END ZP JFK XF JFK4.5
What fare class I'm I looking at here? I tried an Expertflyer search for the days in question and eyeballed the results but didn't see any obvious matches.