Did you ever use the metro in Paris ?
Here some information.
First you must know the difference between the Metro, RER and Train.
Metro lines are managed by the RATP transportation society.
The metro goes in Paris and few cities near by that we call petite couronne.
In the metro pass they correspond to zones 1-2 that means the less expensive price.
RER is a metro that goes more far than the metro but not very long distances, cities that are within (inside) the Ile de France region.
The RER can be managed by the RATP, the metro company, or by the SNCF which is the train company.
Any way they made agreements so when you leave the petite courone wherever you go you'll find 5 zones the further (more far ) you go the more you pay...
For example: Versailles is zone 4 it goes with the RER C that belongs to SNCF.
You will pay more to go there than going to Vincennes Medieval beautiful palace which is in zone 2 with the line 1 of the Metro and with the RER A which belongs to the RATP.
There are RER A and B runned by RATP and RER C, D, and E runned by SNCF whith them you can go quite far to the airport for example.
Finally you have the train which goes really long distances Orléans, Bordeaux, Lille, Reims.
It is runned (managed) by the SNCF.
But that's not in Paris so you do not really need to know about it.