Each of the codes don't have a "meaning", per say, other than their precedence in the codes and certain restrictions for their purchase. In other words, the higher the code (M is higher than H, which is higher than W) the higher the fare normally is.

For Flex, for example, the airline could start selling cheap V fares as far as 6 months out... but with a time limit. The V fares could only be valid until they are sold out or 30 days before a flight, at which time that fare will fully disappear. As one fare bucket sells out or expires (e.g. G) then only tickets at the next class up and above can be sold.

As well, when it comes down to upgrades et. all, the higher the code is the higher in the priority listing you are, for your Altitude status level. So an E50K with an M fare would be upgraded before an E50K with a W fare.

The breakdown of each Flex code (which in reality are Y economy tickets with pre-set fare conditions and discounts) have the above restrictions and discounts.

Either way you would earn 100% of your AQM's from the flight pass, however with an M fare you would now be above all of the other Flex fares when it comes to upgrades. With the usual restrictions for upgrade priority in place, that is.

Edit: Here's an example of AC810 for tomorrow, showing the fare codes as of this minute: J7 C7 D5 Z2 P0 Y9 B9 M6 U6 H6 Q2 V0 W0 S0 T0 L0 K0 R0
As you can see there are no Tango fares left, and the lowest Flex fare would be a Q. If your flight pass only allowed for V fares to be booked, you would not be able to book that flight... while there are still 6 M seats left.