At the end of the day, no matter what you are booking, there are a few main features you wish to know about any fare:
1. How much is it?
2. Can I change or refund my ticket at all?
3. What do I get for my money?
The main criteria, for which airline anyone books, is, of course: "Does the flight get me to where I need to be, when I need to be there?"
No matter how much airlines talk of "differentiation" or anything else for that matter, the above applies to pretty much any trip one is likely to make.
To the above, it seems, there needs to be added a further criteria: "Which airline can I book that it not going to require me to use my maximum personal brain bandwidth or needs me to have a PhD in logic/ statistics and/ or pure maths?"
Air France and KLM, bless them, are helping you decide by introducing no less than seven new booking classes on their medium haul flights. This offers 19 new price points and 6 new economy booking classes, which 6 will apply across the 3 branded fares of light, standard and flex. Confused? Of course not! You are being offered more choice!
There is little difference in reality; as has been since time immemorial, the more an aircraft fills up, the more a seat will cost you. Fares can be changed for 70 Euros plus any fare uplift, you still get a hold bag and you take pot luck on where you sit.
From an agent perspective, the change may be a bit more confusing. Over time, airlines have adopted an informal system to identify cabins and booking classes. W, S, E and T are usually Premium Economy or derivatives thereof; A is a special offer First Class ticket as is, of course F (which denotes full First); P and U are generally used for special offers or by airlines when someone is upgraded; R, D, C, I and J are used for variations on the business class theme. Though these designations are by no means official, they have been generally recognised and used in the manner mentioned - until Air France/ KLM decided to use all sorts of letters which normally apply to other cabins, as their economy class derivatives - the message to all fellow agents is, therefore, watch out!