meFIRST, from ozstamps's Thread listing all known UA revenue and award booking codes and a summary of each one thread:

'R' = XF = First Class - free ticket using miles
'X' = NF = First Class UPGRADE from Business
'O' = XC = Business Class - free ticket using miles
'I' = NC = Business Class UPGRADE from Economy
'N' = XY = Economy Class - free ticket using miles
'E' = Economy Class free ticket (available only to 1K pax)