機倉位置的代號嗎？

What class means depence on which airlines you are with...but usually
they represent the sub-class in each First/Business/Economy class...
Different sub-class have different price/condition/restriction...

Usually the lower the class (Cheaper ticket), there would be more restriction like Monday-Thursday only (As Friday to Sunday demand is higher), no change of date/time, no refund if cancellation, shorter maximum staying time etc... And as the ticket is more expensive, there will be less restriction/condition...
Like if you get a Full fare Economy class ticket which is Y class, your ticket will have more or less no restriction on changing date/time... But 1 restriction is that it will be within 1 year...

When the class of service paid is not available for a particular portion on the route, the passenger will be accommodated in the next available lower class. No refund will be made for this involuntary down grade.

Strightly speaking, there is no difference in term of quality of services onboard between the different sub-class...
What the difference between different sub-class is the terms and condition... As different sub-class would restrict the leight of stopover, the ability of change of date/name, the weight of check-in luggage allows

Different airlines have different meanings.

basically, it's about the conditions imposed onto the ticket - rerouteable? valid on other carriers? flights indicated on ticket only?.....