A FlyerTalk Posting Legend
Join Date: Sep 2009
Location: Minneapolis: DL DM charter 2.3MM
Programs: A3*Gold, SPG Plat, HyattDiamond, MarriottPP, LHW exAccess, ICI, Raffles Amb, NW PE MM, TWA Gold MM
Posts: 102,617
IIRC KLM used to put M on the boarding pass for discounted coach fare classes (usually assigned to seats way in the back, with every seat filled) and use Y for fares that were considered more flexible.