Fare Basis Codes
To add to D582's very detailed post above, here are the subcodes within the fare basis:
EF = Business Flexible
EL = Business Lowest
PF = Premium Economy Flexible
PL = Premium Economy Lowest
LT = Latitude
FL = Flex
TG = Tango
At the moment, my knowledge is that Latitude and higher are status quo with respect to the fare class, so no real worry about the fare basis to figure it out.
However, Flex and Tango, as per D582 is where the basis will tell you whether it's Flex or Tango since they will share the same fare classes.