The letters airlines assign to certain levels of coach can vary widely, but a couple are universal. "Y" class is a widespread denotation for the highest class in coach among most major airlines, according to Jami Counter, senior director of TripAdvisor Flights and a former pricing strategist at American Airlines. These tickets are usually fully refundable, last-minute coach fares purchased mainly by business travelers. They're the most expensive tickets, but they have the most flexibility...
...On the other end of the spectrum, Counter said passengers with the highest-ranked fare basis codes are eligible to get more than the standard miles, sometimes 150 percent, for their flight.