Y will be the cabin of travel, economy, whereas the B or M will be the subclass which corresponds to the fare level applied for each leg of the journey.
To take that a step further, if there are 200 seats in the economy class cabin, the yield management will allocate a certain number to full unrestricted economy fares, and others to more restricted fare levels, youth fares, and so on.
Seems a little strange. I thought there was only one class code for each flight. Wiki says:
Economy class codes Full fare: Y, B Standard fare: M, H, N Special or discount fares: G, K, L, O, Q, S, T, U, V, W, X On most airlines, unrestricted economy ticket is booked as a Y fare. Full fare tickets with restrictions on travel dates, refunds, or advance reservations are commonly classed as B, H, or M, although some airlines may use S, W, or others. Heavily discounted fares, commonly O, T, Q or W, will not permit cabin upgrades, refunds, or reservation changes, may restrict frequent flyer program eligibility, and/or impose other restrictions. Other fare codes such as X are restricted for use by consolidators, group charters, or travel industry professionals. However on some airlines W or X is used for frequent flier program award redemptions.
So I would have expected Y OR B OR M, but not a combination.
The Big Airways website site allows you to check the fare code when booking, and that only gives a single code for each leg.
I confess it has always been something of a mystery to me too.
Tigger, while I too use Wikipedia I *always* take what it says as a guide for more research, not as gospel.
In years gone by Y meant economy class. Then the airlines stated yield management and the system described by Capteonian spread. However, in the minds on many people the idea that Y = economy was (and is) still prevalant. To try and not confuse people (!) some people in airlines and agencies seem to think that if they use codes like Y/M and Y/B people will understand they are booked in economy at the M or B fare level.
And, to go slightly off topic, even airline people don't understand this fare class lark. Some years ago I had a Star Alliance Business Class RTW ticket. I therefore had a ticket where the fare basis for the whole ticket started with C (can't remember all the other letters). But I had been booked on a flight which only had economy class from Kuala Lumpur to Bangkok so the class of travel for that segment was shown as Y. I turned up at KL airport to fly to Bangkok in time for an earlier flight that had Business class and which had space in both Business and Economy available. The staff point plank refused to put me in Business because they said the class of travel for my ticket was Y - they failed to realise the difference between booked and paid fares.