A: Discounted First Class fare. YUP- fares, RTW fares, SWU and Award Upgrades.
 
 B: Basic Economy - highly restricted, no seat choice, no rollaboard, etc. 
 link to FT thread 
 
 C: Business Upgrade - SWU, Award, and some Passenger Sales ('kiosk') Upgrades 
 from Economy to Business. Upgrades from Economy to Business on international 
 flights using certain travel agency certificates.
 
 D: Discounted Business, Round the World Explorer, or Y26nCUP fares
 
 E: Employee travelling on A12 (or A9 - family emergency). Also used to indicate 
 seats available for Confirmed Flight Change within 12 hours of the flight (but 
 passengers are rebooked in their existing inventory class). 
 
 F: Full unrestricted First Class Fare
 
 G: 9th tier Economy fare.  The mysterious G code on American includes 
 federal government Coach. There are two government fares for all 
 federal travelers, they are the YCA and -CA.  The -CA is the 
 capacity controlled government fare (limited number of tickets 
 per flight), which is much cheaper than the YCA and still allows 
 for full refund.  This is the one that would be ticketed as G / GCA.
 
 H: 3rd Tier, discounted Economy Class with few restrictions.
 
 I: Business Class Fare - Discounted Business and group fare, 3rd tier fare. 
 YnCUP-fares.
 
 J: Full fare Business Class.
 
 K: 4th tier Economy; discount refundable changeable fare (e.g. K26d). 
 Also, Military fares
 
 L: 6th tier Economy fare. RTW fares.
 
 M: 5th tier Economy fare. Visit Another Country fares (any carrier), 
 Military fares.
 
 N: 11th tier Economy fare. Visit Another Country fares (alliance); 
 Government and Military fares.
 	
 O: 12th tier Economy fare. Web specials. Consolidator fare typically 
 issued in Europe. 
 
 P: Discounted First Class Fare - Premium transcontinental Flagship 
 service. Domestic first class on 3-class flights. KUP- fares (formerly YUPP).
 
 Q: 12th tier Economy fare. Web specials. Consolidator fare typically 
 issued in Europe.
 
 R: Business Class Upgrade from coach using e-upgrades. EXP UDU, 
 500-mile, and some Passenger Sales / LFBU ('kiosk') Upgrades from 
 Economy to Business on 3-class transcons and 2-class short-haul 
 international flights (e.g., Canada, Mexico, Caribbean, Central America). 
 Operational Upgrades.
 
 S: 10th tier Economy fare. Transatlantic consolidator 2nd tier and 
 package tours fares.
 
 T: Coach / Economy Award.
 
 U: Business Class Award.
 
 V: 8th tier Economy fare. Bereavement fares (?), Transatlantic 
 consolidator 1st tier.
 
 W: Premium Economy fare (previously 7th tier Economy fare).
 
 X: First Class Upgrade.
 
 X: Business Class Upgrade - EXP UDU and 500-mile Upgrades from 
 Economy to First on 2-class domestic U.S. flights. Upgrades from 
 Business to First on 3-class international flights using certain travel 
 agency certificates. Operational Upgrades.
 
 Y:.1st Tier, Full Fare unrestricted Economy class.
 
 Z: First Class Award.
 
 NA: Plan AAhead Award travel?
 
 AA Economy fare codes order: Y, H, K, M, V, Q, S, N, L, O and B
 
 Fare codes sorted by type:
 
 Coach excursion fares are: H, K, M, V, N, L, Q
 Basic Economy: B
 Employees: E
 Government: Y/L
 Youth/student international: H/K
 Military: H/M 
 Promotional (e.g. VUSA - Visit USA): L
 NetsAAvers: L
 Bereavement: M
 Consolidator: K, W, V, L, O
 Air/sea: W, K
 RTW: M (coach), D (business), A (first)

 Here's my understanding so far. Parts may be inaccurate or 
 not apply to all fares.
 
 first letter: booking class (except for UP fares)
 
 first number (other than 26): domestic: advance purchase 
 requirement. if it is 0, same day ticketing is allowed, waitlisting is not.
 second number: minimum stay requirement
 third number: maximum stay requirement
 
 A: one-way
 R: round-trip
 
 AP: advance purchase
 BIZ/BZN: instant purchase non-refundable fares for 
 business travelers--no Saturday night stay requirement, 
 30-day maximum stay
 FS: flight-specific
 J/JFK: JFK only
 N/NR: non-refundable
 Z: restricted, which may mean confirmation required 
 (possibly refundable though)
 AA: Web-only fare
 UP: instant upgrade into different booking class
 
 GB: originate in GB, following number is year of fare applicability
 US: originate in US
 
 E: minimum stay requirement (Saturday unless overridden 
 by second number)
 M: Saturday stay requirement (second number becomes 
 maximum stay)
 C: next number is change fee
 
 second position:
 P: peak season
 H: high season
 K: shoulder season
 L: low season
 M: other seasonal restriction
 
 third position:
 W: Sun 12:01a-Mon 11:59a, Thu noon-Sat 11:59a
 X: not on Fri or Sun
 B: weekend (Thu/Fri-Mon, no Sun with X)
 S: midweek (Tue-Thu or Tue-Wed, also Mon with X)
 F: specific times (e.g. daily 8:45a-9:45a)
 O: specific times and dates (e.g. Tue/Wed 8:45a-9:45a)
 
 A: advance ticketing required
 R: reservation confirmation required
 D: ??? (appears on shorter flights)
 
 last position:
 S: fuel surcharge
 G: restricted day of first travel
 X: restricted day of first travel