ExpertFlyer lists the following NW codes below. "I" isn't among them so it clearly has something to do with the 2% club.


A NW Domestic - First - Award Select
AN NW FIRST CLASS Select
B NW ECONOMY Select
BN NW ECONOMY Select
C NW BUSINESS Select
CN NW BUSINESS Select
D NW Int'l - Business - Award Select
DN NW DELUXE NIGHT Select
E NW SHUTTLE SERVICE Select
F NW FIRST CLASS Select
FN NW DELUXE NIGHT Select
H NW ECONOMY Select
HN NW ECONOMY Select
J NW BUSINESS Select
K NW ECONOMY Select
KN NW ECONOMY Select
L NW ECONOMY Select
M NW ECONOMY Select
N NW ECONOMY Select
P NW FIRST CLASS Select
Q NW ECONOMY Select
R NW Domestic - Upgrade to First for WP members and using miles Select
S NW STANDARD Select
T NW ECONOMY Select
U NW Int'l Mileage Upgrade to Business from Discounted Coach Select
V NW ECONOMY Select
W NW Domestic & Int'l - Coach - Award Select
X NW ECONOMY Select
Y NW ECONOMY Select
YN NW ECONOMY Select
Z NW Int'l Mileage Upgrade to Business from Full Fare Coach Select