Edit: I see where my confusion is originating "two train stations on the RER B line." I don't mean to wield a red pen but we usually say RER station, even if the Aeroport de Paris signs say "Paris by train" although they mean RER--their bad.
Then the Aeroport de Paris and RER administrations further complicated the situation by calling the stop below Terminal 3, CDG 1, while there is a (closed) Terminal 1 in a different building. And then I totally forgot about the RER station in T2, although I used it just a month ago, flying to the US--my bad. Confusion all around.
So for anyone who has stumbled on our jumble, here is an attempt to distinguish the transportation at CDG, which is enough to spin anyone's head, especially with so many similar names:
CDGval is the internal tram system within the airport.
RER in T3 and T2 is the transportation system that circles greater Paris with lines A and B going through. This regional system goes into the Paris suburbs and sometimes a bit beyond. The RER stations are in Terminals 2 and 3. Terminal 1 is still closed.
Trains from the train station in Terminal 2:
TER are local trains from the SNCF train station. I've never paid attention if any TER or Intercity trains leave from CDG. However, some regional TER trains that leave from Paris city train stations go to neighboring regions such as Burgundy, the Loire, etc.
TGV high-speed trains leave from the CDG train station to Brussels, the Mediterranean region, Brittany, etc.