How does Google Flights define classes?

What criteria do Google Flights use to classify different products across airlines?

The airlines themselves provide the definition. It is not up to the travel agents (online or otherwise) to redefine the cabin classes. I imagine such re-interpretations would be a violation of a TA's agency agreement with the airline: the travel agent is an agent for the airline, contractually obliged to operate in accordance with the airline's wishes. It is not a free shop that can market goods bought in bulk however it sees fit.

Network airlines file their fares (and much more information) centrally either with ATPCO, the Airline Tariff Publishing Company, or with SITA, le Société Internationale de Télécommunications Aéronautiques. Some of the larger airlines file with both, but ATPCO is by far the dominant publisher. (To my knowledge, there are no other competitors, and each is actually owned by a conglomerate of airlines, as industry bodies, not freely trading companies.)

Among fare tariffs, service fees, frequent flyer status maps and other information, the airlines may also use this service to define how their fare classes map to cabins.

These two companies then publish their data on to the Global Distribution Systems such as Amadeus or Sabre (and other interested parties, for a substantial fee). A GDS combines all these delayed data with live availability information and a direct reservation feed to each airline, allowing travel agents to price itineraries, reserve space, and issue tickets.

I will now ignore SITA since I have little experience with them.

ATPCO provides a Reservation Booking Designator by Cabin Answer Table. It describes the table in the documentation thus.

The RBD Answer table product provides subscribers with a listing of carriers’ RBDs and their associated cabin (Premium First, First, Premium Business, Business, Premium Economy, and Economy). A carrier’s RBD Answer Table may consist of several sequences with a listing of RBDs and their associated cabin that apply only if specific global indicator, geographic locale, travel/ticketing dates flight numbers and equipment are matched.

Like most of ATPCO's feeds, it's a simple fixed-width text file, in a format last updated in the early 90s, compressed with gzip and pushed once per day over FTP to each customer, either complete, or in differential form (only the differences since yesterday) if the customer prefers.

It's a flexible format in some respects, allowing a particular fare booking code (or "reservation booking designator") such as Y to define one of several pre-specified cabins. That mapping can be restricted by airline (obviously), tariff (such as "AT" for transatlantic), equipment (such as "744" for a 747-400), specific flight number, city pair, country pair, travel date, and a few other conditions, but most airlines don't refine on these parameters beyond the booking code.

On the other hand, it's inflexible in that the only allowed cabin designators are

• Premium First
• First
• Premium Business
• Business
• Premium Economy
• Economy

As an example of this limitation, BA used to have R class for supersonic class, but that sadly is no longer the case. I believe it was branded as business in the GDS.

However, airlines can now arrange for a further distinction using "Branded fares". This allows arbitrary names to be given to cabins. For instance, BA brands its business services as "Club Europe" (shorthaul intra-Europe), "Club World" (intercontinentally) and "Club" (shorthaul within Southern Africa). This is not universally supported.

If the airline declines to provide this information, then ATPCO instructs that you should use the default definitions specified in IATA Resolution 728. This says that P, F, and A is first class; J, C, D, I, and Z is business; W is premium economy; and S, Y, H, B, K, L, M, N, Q, T, V, and X are economy. (It's actually refined a bit further because historically IATA's "any airline" YY fares were priced for different buckets, but the commercial need for that has probably gone now.) E, G, O, R, and U are left unspecified, but ATPCO documentation states they should be treated as economy if the marketing carrier did not provide an alternative description.

Sometimes we see mistakes like Aer Lingus F fares being described as first class. Examination of the RBD answer table shows that this should be considered economy.