What do these train codes on Paris RER B (airports subway line) mean?

Question

While standing on the platform waiting for the RER B train to Charles de Gaulle airport in Paris, I noticed codes like ELOT or EPEX besides arrival time, platform number, destination, etc.

What do these codes mean?

I didn't see any information in the station. RER B has two types of trains, one that stops at every station on the way and an express one that stops at a few stations (Gare du Nord, etc) before Charles de Gaulle airport. Can express trains be distinguished from these codes?

Note: the codes above are given as examples. Not sure they are accurate as I do this out of memory a few days after traveling in Paris. The second one I saw ended in EX but was not an express train.

Answer 1

These train codes are called mission codes.

https://fr.wikipedia.org/wiki/Ligne_B_du_RER_d%27%C3%8Ele-de-France, via Gtranslate

• First letter (train destination): The first letter provides information on the final destination of the train, according to the two tables below.
• Second letter (serving): The second letter indicates the stations served by the train.
• Third and fourth letters: They are chosen to make everything pronounceable and memorable.

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Meaning of first letter used regularly:

Letter	Destination
E	Aéroport Charles-de-Gaulle 2 TGV
I	Mitry - Claye
J	Denfert-Rochereau
K	Massy - Palaiseau
L	Orsay-Ville
P	Saint-Rémy-lès-Chevreuse
Q	La Plaine - Stade de France
S	Robinson
U	Laplace

Meaning of first letter used exceptionally:

Letter	Destination
A	Gare du Nord
G	Aulnay-sous-Bois
H	Bourg-la-Reine
M	Châtelet - Les Halles
V	Lozère
X	Parc des Expositions
Y	Les Baconnets

All codes from a few years ago: http://michou94.free.fr/html/missiona.html.

Answer 2

The 4-letters codes describe the different routes and types of trains (express or not) of the RER B line. Indeed, RER B forks at Aulnay-sous-Bois station (while traveling to the North-East part of Paris suburb) with the B3 route going to Charles-de-Gaulle airport and the B5 route going to Mitry-Claye station. It also forks in the opposite direction at Bourg-la-Reine station with the B4 route going to Saint-Remy-les-Chevreuses.

This article describes it:

The fastest trains from CDG to central Paris have route codes beginning with KA (KALE, KALI, KAPE, KARE, etc.) or JA (JAMI, JANO), or UL. Other trains make more stops.

For your return to CDG airport from central Paris, look for B3 trains with route codes beginning with E, because all trains beginning with E go to CDG. The express trains are ERIC, EROC, ERIO, ERSE, EDME [,or] EDDY.

Here is a picture from the same article (direction CDG - Central Paris)

It seems that JIBY train stops at Denfert-Rochereau and does not go all the way, in particular not stopping at Antony (near Orly airport) and KDIT stops on the B4 route at Massy-Palaiseau.

Additional details in this article