The only time the fare class / code comes in to play is in the case of full fare Economy Class tickets which take priority within each status group. (travelingformiles.com)
First, it can give you a complimentary upgrade to business class if you book a full fare economy class ticket. (upgradedpoints.com)
The issue with these is that these full fare economy tickets can be very expensive, so it's just like you're paying for business class. (uponarriving.com)