It's not that clear, and it can change - and this pertains only to AA. I'll take a stab here: so far, as I understand it - and others will correct or add, and I am modifying this as they do (THANKS, FT colleagues!)

FIRST CLASS
F = Full price First Class
A = Discount First Class with RTW (Round The World ticket,) upgraded using VIP, miles (and I've had some YUPs book here.)
P = First Class on three-class domestic a/c (767-223ERs,) YUPP upgrades to First
X = 500-miles or comp EXP upgrade domestic First Class
Z = Award First Class (no miles / points earned)

BUSINESS CLASS
J = Full price Business Class
D = Discount Business Class / Business and RTW tickets
C = Upgraded to Business using VIPs, miles...
I = Highly discounted Business Class, lots of conditions, not upgradable
R = 500-miles or comp EXP upgrade for "North American" Business Class upgrades (all two-class flights to Caribbean, Central America, Canada and Mexico, First is labelled Business and so X becomes R)
U = Award Business Class (no miles / points earned)

ECONOMY CLASS ("Coach")
Y = Full fare Economy, no conditions
B = "Full fare" Economy, a few conditions
YUP / YUPP = Domestic economy fare that books into next class of service when seats are available
-------------------------------------------- ALL paid above earn 1.5 EQP per EQM, except upgraded seats generally earn at the original paid fare level.

H, K, M, L, W and V = Discount Economy Class fares with various conditions, restrictions - earn 1 EQP per EQM. "L" is generally used for RTW tickets in Economy.

G, Q**, N, O** and S = Deep Discount Economy fares with lots of conditions, earn 0.5 EQP per EQM, except as noted

** Tickets between North America and Europe, India, Asia and Latin America booked in O and Transatlantic tickets booked in Q earn no miles or points, are generally iirc not upgradable under conditions where they do not earn miles. (They DO earn miles and 0.5 EQM "domestically," meaning North America, Mexico, Central America, Caribbean... and generally can be upgraded when they are upgradable with 500 mile "sticker" upgrades.)

T = Award Economy (no miles, no points)

The best way to learn about the specific conditions is to click on "Detailed Fare Rules" before buying the ticket, or, if one has done that already and has purchased via aa.com, one can go to https://www.refunds.aa.com/ and enter the ticket information; the site allows you to see the detailed fare rules before requesting the actual refund.

Note: This is NOT exhaustive, and these codes will vary by airline - e.g. "T" on BA is World Traveller Plus, a whole different animal. This specifically relates to AA.

If this comes together and the veterans vet it , I can add this to the AA Wiki for us.