It looks like they are aligning with AA fare codes. On AA, there are two upgrades code for F: one for instruments like an eVIP/SWU (that's A class on AA) and one for the standard domestic upgrades that are free to EXPs and require 500-miles upgrades (aka stickers) for PLTs and GLDs (that's X class on AA, which AA made EF stop reporting on a while ago).
And then C is the e-VIP/SWU upgrade inventory class for Business.
I could be wrong, but this new chart certainly looks very familiar to me as a pmAA flyer.