Coach class fares fluctuaterna lot, so on any one flight there could be fares ranging from $59 torn$459. Obviously, the $59 does not have the same benefits as a $459rnticket, so they use different letters to designate this. The higher thernfare code, the more flexibility you get for changing your ticket,rncancelling your flight for a refund, getting an upgrade or even gettingrnmore frequent flier miles.
For example, Person A and Person Brnare both travelling on flight 928. Person A paid $513 and is travellingrnin "Y" class, and Person B paid $133 and is travelling in "H" class.rnThe person in "Y" class has a lot more flexibility. He can changernflights, get a full refund, might accruernmore frequent flier miles and has a better chance of getting upgradedrnto Business or First. The person in "H" class might have to pay a hugernfee to change his flight, will not get a full refund, will get 100% orrnless frequent flier accrual and has almost no chance of getting upgraded to first class.
Forrnmost people, those letters really don't matter. After all, there is arnhigh chance that Person A and Person B will sit right next to eachrnother on the plane. It really only matters to the airline staff.
This site explains different fare codes AA
and other OneWorld partners use.