R: Business class Non-Rev (Swiss only!)

Premium Economy Class (not available on Swiss, but reserved as OS and LH use it):
(In the above table E and G are still listed in Economy but the letters are not available anymore. for booking anymo! Hence you can't get booked into this class on Swiss. They are unused and reserved for premium Economy)

E: Reserved for Premium Economy
G: Reserved for Premium Economy
N: Discount Premium Economy (Used for Non-Rev Economy Flights on Swiss only!)

R: Premium Economy Class Award (Used for Non-Rev Business Flights on Swiss only!)

Economy Class:

Y: Full Fare unrestricted Economy Class
B: Almost Full Fare minimal restricted Economy Class

M: Almost Full Economy Fare with few restrictions
U: Higher Level Economy Fare with few restrictions
H: Economy Fare with moderate restrictions

Q: Discount Economy Fare with moderate restrictions, used for RTW trips
V: Discount Fare Economy, often with a lot of restrictions
W: Discount Fare Economy often with many restrictions
S: Ready-To-Fly Economy Fare

T: Heavy Discount Fare Economy Class with a lot of restrictions
L: Heavy Discount Economy Fare with lots of restrictions
K: Heavy Discount Economy Fare with lots of restrictions

X: Economy Class Award