Indeed, you can also see the booking class for OAL, which is useful when buying a multi-carrier through fare. For example, if you were flying, say, NYC to SGN, UA partners with a laundry list of airlines on the last leg; here's the first (of 12) routings for the cheapest K fare on the date I searched:

If you wanted to know which fare class the last leg would book into on SQ or BR or CX or whatever, you'd enter that airline when you clicked on the 'C'.

I'm still curious how you bought this fare in the first place.