Paris Travel Guide - HTML5

trains make stops at all stations on a given line.  Four-letter-codes (KRIN, DIPA, TORE, etc.) are used for the RER and Transilien trains.  on RER A, B and C the first letter indicates the destination of the train, the second the branch or service type, and the last two are to make the name easier to memorize  on RER D and E, the first letter is destination, the second letter is service type, the third letter is branch, and the fourth letter is direction  on Transilien lines, it's usually one name for every service type.

You can look up what these codes mean