Deciphering Air Canada Fare Codes

The first letter is commonly called the booking class. It defines which bucket of seat availability there needs to be to be bookable. If you use something like Expert Flyer, or one of the other paid availability tools, you can check what's available on a given flight.

The remainder is the full fare details

I've just looked up a random Air Canada domestic return flight for next week, and the cheapest tickets showing are K18ZTGA. That K tells us that to be valid, amongst the other restrictions, there needs to be seats in the K booking class. As you get closer to the flight, airlines tend to zero out the cheaper booking classes, which allows them to effectively make the cheaper fares unavailable

Using the ITA Matrix, doing a search, and clicking through yields the full details for K18ZTGA for that route:

Air Canada (AC) K18ZTGA YTO to YMQ
General notes

ONE WAY INSTANT PURCHASE FARES            K
 FOR ONE WAY FARES

Category 2: Day/Time restrictions

  FROM YMQ -
    PERMITTED 1000AM TO 430PM OR 501PM TO 530PM OR 601PM TO
    630PM OR 801PM TO 830PM OR 900PM TO 1000PM DAILY.
  TO YMQ -
    PERMITTED 1000AM TO 430PM OR 501PM TO 530PM OR 601PM TO
    630PM DAILY.

Category 5: Advanced reservation/ticketing restrictions

  RESERVATIONS FOR ALL SECTORS ARE REQUIRED AT LEAST 18 DAYS
  BEFORE DEPARTURE.
  WAITLIST AND STANDBY NOT PERMITTED.
  TICKETING MUST BE COMPLETED WITHIN 1 DAY AFTER
  RESERVATIONS ARE MADE OR AT LEAST 18 DAYS BEFORE DEPARTURE
  WHICHEVER IS EARLIER.
         NOTE -
          OPEN SEGMENTS NOT PERMITTED.
          --------------------------------------------------
          DUE TO AUTOMATED TICKETING DEADLINE CONTROL
          DIFFERENCES COULD EXIST BETWEEN THE FARE RULE
          LAST TICKETING DATE AND THE FLIGHT
          FIRMING GENERATED TICKETING DEADLINE MESSAGE -
          THE MOST RESTRICTIVE TICKETING DEADLINE APPLIES.

Category 8: Stopover restrictions

  IF THE FARE COMPONENT INCLUDES TRAVEL VIA YTO
  AND - 530AM TO 559AM DAILY
    NO STOPOVERS PERMITTED.
       NO STOPOVER OCCURS IF PASSENGER TAKES NEXT AVAILABLE
        FLIGHT WITHIN 1 HOUR.

Category 10: Combinability

   APPLICABLE ADD-ON CONSTRUCTION IS ADDRESSED IN
   MISCELLANEOUS PROVISIONS - CATEGORY 23.
  OPEN JAWS
    FARES MAY BE COMBINED ON A HALF ROUND TRIP BASIS
    -TO FORM SINGLE OR DOUBLE OPEN JAWS.
   PROVIDED -
     WHEN THE OPEN SEGMENT OCCURS
      -WITHIN AREA 1
       COMBINATIONS ARE WITH ANY FARE FOR CARRIER AC/UA/5T/
     7F/3J IN ANY RULE AND TARIFF.
  2-COMPONENT CIRCLE TRIPS/MULTI-COMPONENT CIRCLE TRIPS
    FARES MAY BE COMBINED ON A HALF ROUND TRIP BASIS
    -TO FORM 2-COMPONENT CIRCLE TRIPS/MULTI-COMPONENT CIRCLE
    TRIPS.
   PROVIDED -
     COMBINATIONS ARE WITH ANY FARE FOR CARRIER AC/UA/5T/7F/
     3J IN ANY RULE AND TARIFF.
  END-ON-END
    END-ON-END COMBINATIONS PERMITTED. VALIDATE ALL FARE
    COMPONENTS. FARES MUST BE SHOWN SEPARATELY ON THE
    TICKET. TRAVEL MUST BE VIA THE POINT OF COMBINATION.
    SIDE TRIPS PERMITTED.
   PROVIDED -
     COMBINATIONS ARE NOT FOR CARRIER WS
  OTHERWISE
    END-ON-END COMBINATIONS PERMITTED. VALIDATE ALL FARE
    COMPONENTS. FARES MUST BE SHOWN SEPARATELY ON THE
    TICKET. TRAVEL MUST BE VIA THE POINT OF COMBINATION.
    SIDE TRIPS PERMITTED.
 NOTE - THE CITY PAIRS BELOW ARE CONSIDERED THE SAME POINT-
        YYZ-YTZ

Category 12: Surcharges

  IF INFANT UNDER 02 WITHOUT A SEAT.
    THERE IS NO CHARGE FOR TRAVEL.
  THE PROVISIONS BELOW APPLY ONLY AS FOLLOWS -
  TICKETS MAY ONLY BE SOLD IN CANADA/THE UNITED STATES.
    A SURCHARGE OF CAD 3.00 WILL BE ADDED TO THE APPLICABLE
    FARE FOR TRAVEL.
         NOTE -
          THIS IS AN INSURANCE SURCHARGE.  THE SURCHARGE
          APPLIES IN ADDITION TO ALL OTHER CHARGES AND IS
          NOT SUBJECT TO ANY DISCOUNT.
    AND - A SURCHARGE OF CAD 3.00 WILL BE ADDED TO THE
          APPLICABLE FARE FOR TRAVEL.
         NOTE -
          THIS IS A NAVIGATION SURCHARGE. THE SURCHARGE
          APPLIES IN ADDITION TO ALL OTHER CHARGES AND IS
          NOT SUBJECT TO ANY DISCOUNT.
  THE PROVISIONS BELOW APPLY ONLY AS FOLLOWS -
  TICKETS MAY NOT BE SOLD IN CANADA/THE UNITED STATES.
    A SURCHARGE OF CAD 3.00 WILL BE ADDED TO THE APPLICABLE
    FARE FOR TRAVEL.
         NOTE -
          THIS IS AN INSURANCE SURCHARGE.  THE SURCHARGE
          APPLIES IN ADDITION TO ALL OTHER CHARGES AND IS
          NOT SUBJECT TO ANY DISCOUNT.
    AND - A SURCHARGE OF CAD 3.00 WILL BE ADDED TO THE
          APPLICABLE FARE FOR TRAVEL.
         NOTE -
          THIS IS A NAVIGATION SURCHARGE. THE SURCHARGE
          APPLIES IN ADDITION TO ALL OTHER CHARGES AND IS
          NOT SUBJECT TO ANY DISCOUNT.
    AND - A SURCHARGE OF CAD 50.00 WILL BE ADDED TO THE
          APPLICABLE FARE FOR TRAVEL.
         NOTE -
          THIS IS A TICKETING SURCHARGE.  THE SURCHARGE
          APPLIES IN ADDITION TO ALL OTHER CHARGES AND IS
          NOT SUBJECT TO ANY DISCOUNT.

Category 14: Travel restrictions

  TRAVEL ON THIS FARE COMPONENT MUST COMMENCE BY MIDNIGHT ON
  31OCT 15.

Category 15: Sales restrictions

  TICKETS MUST BE ISSUED ON/BEFORE 27AUG 15.

Category 15: Sales restrictions

  SALE IS RESTRICTED TO SPECIFIC AGENTS.
  TICKETS MAY BE ISSUED BY ELECTRONIC TICKETING.
         NOTE -
          -PAPER TICKET MAY BE ISSUED FOR OAL CARRIERS NOT
           INVOLVED IN E TICKETING WITH AC.
          -PAPER TICKET MAY BE ISSUED FOR ITINERARIES WHERE
          ELECTRONIC TICKETING IS NOT SUPPORTED.
  OR - TICKETS MUST BE ISSUED BY ELECTRONIC TICKETING.
       TICKETS MAY NOT BE SOLD IN VENEZUELA/GREECE.
       TICKETS MAY ONLY BE SOLD IN AREA 1/AREA 2/AREA 3.
         NOTE -
          -PAPER TICKET MAY BE ISSUED FOR OAL CARRIERS NOT
           INVOLVED IN E TICKETING WITH AC.
          -PAPER TICKET MAY BE ISSUED FOR ITINERARIES WHERE
          ELECTRONIC TICKETING IS NOT SUPPORTED.

Category 16: Penalties

  CANCELLATIONS
    TICKET IS NON-REFUNDABLE.
         NOTE -
          --------------------------------------------------
          -TANGO TICKET/VALIDITY FORFEIT POLICY-
          FULLY UNUSED TICKETS MAY BE REBOOKED AND TRAVEL
          COMMENCED UP TO ONE YEAR FROM ORIGINAL ISSUE DATE.
          PARTIALLY USED TICKET MAY BE REBOOKED AND TRAVEL
          COMPLETED UP TO ONE YEAR FROM ORIGINAL OUTBOUND
          DATE.
          FLIGHT SEGMENT OR SEGMENTS MUST BE CANCELLED ON OR
          PRIOR TO ORIGINAL DEPARTURE DATE. FAILURE TO
          CANCEL FLIGHT SEGMENT OR SEGMENTS PRIOR TO
          DEPARTURE DATE WILL RESULT IN COUPON OR COUPONS
          FORFEIT AND COUPON WILL HAVE NO VALUE.
          --------------------------------------------------
          WAIVED FOR THE DEATH OF THE PASSENGER OR PASSENGER
          FAMILY MEMBER OR PASSENGERS TRAVELLING COMPANION.
          DOCUMENTATION REQUIRED.
          IN THE EVENT PSGRS ARE CALLED TO JURY DUTY OR ARE
          SUBPOENAED FULL REFUND WILL APPLY UPON
          PRESENTATION OF JURY SUMMONS OR SUBPOENA PROVIDED
          FLIGHT SEGMENTS ARE CANCELLED PRIOR TO TICKETED
          DEPARTURE DATE.
  CHANGES
    ANY TIME
      CHARGE CAD 75.00 FOR REISSUE/REVALIDATION.
         NOTE -
          --------------------------------------------------
          FOR CHANGES TO ONLINE MULTI FARE COMPONENT ONLY
          ONE CHANGE FEE APPLIES PER DIRECTION.
          CHANGE FEE-
          COLLECT CAD75 FOR VOLUNTARY CHANGES TO ANY FLIGHT
          SEGMENTS IN ACCORDANCE WITH THE FOLLOWING-
          A. THE NEW FLIGHT SEGMENT MUST BE REPRICED AND
          REISSUED WITH CURRENT AND APPLICABLE FARE.
          B. IF THE NEW FLIGHT SEGMENT IS OF EQUAL OR
          HIGHER VALUE THE FARE DIFFERENCE MUST BE PAID.
          C. IF NEW FLIGHT SEGMENT IS OF LOWER VALUE THE
          FARE DIFFERENCE IS LOST AND NO REFUND WILL APPLY.
          D. FLIGHT COUPONS MUST BE USED IN SEQUENCE.
          CHANGES IN THE SEQUENCE OF ANY FLIGHT COUPON WILL
          RESULT IN A TICKET REISSUE.
          -PLEASE NOTE-
          IF A FARE IS COMBINED WITH HALF A ROUND TRIP FARE
          IN A CIRCLE TRIP - THE HIGHEST CHANGE FEE AND THE
          MOST RESTRICTIVE FARE RULE APPLIES TO THE ENTIRE
          TICKET.
          --------------------------------------------------
          NAME CHANGES - FEE OF CAD 100.00
          PERMITTED ONLY FOR FULLY UNUSED TICKETS ON
          014 TICKET STOCK ON AC - AC ROUGE AND
          AC EXPRESS OPERATED FLIGHTS.
          REISSUE MUST BE DONE BY AC CALL CENTERS ONLY.
          FOR BOOKINGS MADE WITH AC DIRECTLY - CONTACT AC
          ANYTIME.
          FOR BOOKINGS MADE WITH TRAVEL AGENCY - CONTACT AC
          WITHIN 72HRS OF DEPARTURE.
          NOT PERMITTED FOR PARTIALLY USED TICKETS.
          NOT PERMITTED FOR TICKETS WITH INTERNATIONAL
          ITINERARIES.
          NOT PERMITTED FOR TICKETS WITH AC FLIGHTS
          OPERATED BY OTHER CARRIERS.
          --------------------------------------------------
          AIRPORT SAME DAY CONFIRMED CHANGE FEE-
          FOR TANGO -TG TYPE FARES ON ROUTES BETWEEN
          YOW/YMQ-YTO AND BETWEEN YYC/YEA/YVR PASSENGERS
          HOLDING CONFIRMED RESERVATIONS AND
          TICKETS MAY CONFIRM EARLIER/LATER SAME DAY FLIGHTS
          BETWEEN THE SAME ORIGIN/DESTINATION/STOPOVER
          POINTS FOR A NONREFUNDABLE FEE OF CAD1O0/USD100
          OR MAY STANDBY ON EARLIER SAME DAY FLIGHTS
          BETWEEN THE SAME ORIGIN/DESTINATION AT NO CHARGE.
          FOR ANY OTHER ROUTES ON TANGO -TG TYPE FARES
          PASSENGERS HOLDING CONFIRMED RESERVATIONS AND
          TICKETS MAY CONFIRM EARLIER/LATER SAME DAY
          FLIGHTS FOR A NON REFUNDABLE FEE OF CAD150/USD150.
          NOTE-
          A. SEAT INVENTORY MUST BE AVAILABLE IN THE CABIN
          APPLICABLE TO THE FARE CHARGED.
          B. POLICY APPLIES FOR PASSENGERS NEXT OUTBOUND OR
          INBOUND ONLY. CHANGES CANNOT BE MADE TO INBOUND
          SEGMENTS BEFORE THE ORIGINAL TICKTED OUTBOUND HAS
          BEEN FLOWN.
          C. CONFIRMED CHANGE MAY ONLY BE MADE AT THE
          AIRPORT ON THE DAY OF THE FLIGHT AND PRIOR TO THE
          ORIGINAL DEPARTURE TIME.
          D. REGULAR FARE RULES APPLY - CHANGE FEES/ADCOL-
          FOR ANY CHANGES WHICH DO NOT QUALIFY FOR THE
          AIRPORT SAME DAY CONFIRMED CHANGE FEE.
          E. SAMEDAY STANDBY ONLY PERMITTED ON ROUTES
          BETWEEN YOW/YMQ-YTO AND BETWEEN YYC/YEA/YVR.
          --------------------------------------------------
          TICKET UPGRADE - PASSENGERS UPGRADING TO A
          BUSINESS CLASS FARE J-/C-/D-/Z-/P- OR PREMIUM
          ECONOMY FARES O-/E- WILL NOT BE
          ASSESSED THE CHANGE FEE.
          ALL OTHER RULES MUST BE MET.
          ANY NONREFUNDABLE AMOUNT REMAINS NONREFUNDABLE.
          --------------------------------------------------
          CHANGE FEES ARE SUBJECT TO THE SAME TAXES AS THE
          ORIGINAL TICKET.

Category 18: Endors*m*nts

  THE ORIGINAL TICKET MUST BE ANNOTATED - AC ONLY/NON-REF/
  CHGE FEE - IN THE ENDORSEMENT BOX.

Category 19: Discounts

  1ST INFANT UNDER 2 WITHOUT A SEAT - NO CHARGE
    MUST BE ACCOMPANIED ON ALL FLIGHTS IN SAME COMPARTMENT
      BY ADULT 16 OR OLDER
  OR - 1ST INFANT UNDER 2 WITH A SEAT - CHARGE 100 PERCENT
         OF THE FARE.
             TICKET DESIGNATOR - INF100.
         MUST BE ACCOMPANIED ON ALL FLIGHTS IN SAME
           COMPARTMENT BY ADULT 16 OR OLDER.

Category 22: Other Discounts

         NOTE -
          CONTACT CARRIER FOR FURTHER INFOS

Category 31: Voluntary changes

  IN THE EVENT OF CHANGES TO TICKETED FLIGHTS
   ANYTIME WITHIN TKT VALIDITY
    CERTAIN DOMESTIC REISSUE PROVISIONS MAY BE OVERRIDDEN BY
    THOSE OF AC INTERNATIONAL FARES
     CHARGE CAD 75.00 FOR REISSUE OR HIGHEST FEE OF ALL
     CHANGED FARE COMPONENTS AND
      REPRICE
      A. CHANGED FARE COMPONENTS USE FARES IN EFFECT TODAY
      B. ALL OTHERS USE CURRENTLY TKTD FARE
       PROVIDED ALL OF THE FOLLOWING CONDITIONS ARE MET-
       1. NO CHANGE TO FARE BREAKS UP TO THE FIRST CHANGED
       FARE COMPONENT
       2. WHEN NO INTL COUPONS REMAIN - ALL NEW TRAVEL MUST
       BE DOMESTIC
       3. FULLY FLOWN FARE NOT REPRICED TO FURTHER POINT
       4. AC FARES ARE USED
       5. ALL RULE AND BOOKING CODE PROVISIONS ARE MET
       6. ADV RES IS MEASURED FROM NEW TKT ISSUE DATE IF
       CURRENT FARES/FROM PREVIOUS TKT ISSUE DATE IF
       HISTORICAL FARES TO DEPARTURE OF PRICING UNIT
       7. TKT REISSUE RESTRICTED TO AC
   WHEN CHANGE RESULTS IN LOWER FARE IGNORE RESIDUAL THEN
   ADD-COLLECT
   ENDORSEMENT BOX- HIGHER NON-REF AMOUNT.

Category 50: Application

TANGO FARE
 APPLICATION
   CLASS OF SERVICE
     THESE FARES APPLY FOR ECONOMY CLASS SERVICE.
 CAPACITY LIMITATIONS
   CARRIER SHALL LIMIT THE NUMBER OF PASSENGERS
   CARRIED ON ANY ONE FLIGHT AT FARES GOVERNED BY
   RULES MAKING REFERENCE HERETO AND SUCH FARES
   WILL NOT NECESSARILY BE AVAILABLE ON ALL FLIGHTS
   OPERATED BY THE CARRIER.  THE NUMBER OF SEATS
   WHICH THE CARRIER SHALL MAKE AVAILABLE ON A
   GIVEN FLIGHT WILL BE DETERMINED BY THE CARRIERS
   BEST JUDGEMENT AS TO THE ANTICIPATED TOTAL
   PASSENGER LOAD ON EACH FLIGHT.
 RULES NOT APPLICABLE
   165/170/175
 OTHER CONDITIONS
   WAITLISTING IS NOT PERMITTED.

Some airlines assign the code randomly, others have certain naming conventions, but basically you need to look up the actual rules to see what they mean!