"C" is good. I fly NW on upgraded NW tickets using OnePass miles a lot to Asia. As CO issues these NW tickets, the code for WBC is all over the lot. The last one was "D" which is suppsoed to be for a free WBC ticket, meaning I had to send in the stubbs to get the miles credited. I have also seen M and C. C is the best, since if you miss a connection, in the chaos of rebooking people, NW will generally miss youy paid a low upgraded fare and rebook you on any airline in C class (This happened when we missed a connection in NRT and NW put us in C on MH). If they see something other than C, they may well try to put you in coach for a missed connection on another carrier.