will be travelling from Charles de Gaulle Airport to Denfert Rochereau and then transferring to the metro line 4 to travel to Barbara Station. I’m confused about the zones. The airport train costs 11.80EUR but does not include metro line ticket. So I will need to purchase a metro line ticket when transiting through Denfert. The app says the ticket is 2.15EUR but this looks to be for zone 1 only. According to the transport map i will be going through zone 1 and 2 to reach my destination. Can you please advise?
The standard CDG airport train ticket to Paris will include transfers to Metro made inside central Paris, such as your transfer from RER B to Metro 4 at Denfert Rochereau station (central Paris is zone 1). So the final price will be 11.80€, including your transfer onto Metro to Barbara. You won't need an additional Paris Metro ticket to complete your journey.
The confusing thing about Paris Metro is that zones don't apply to the Metro network; zones are for the RER train network.
The Paris Metro system is all one contiguous system without zones, even if Metro stations are located in RER zones 2 and 3. This is why you can transfer from RER onto Metro at Denfert Rochereau and be covered through to Barbara station (in RER zone 3), since you made your transfer to Metro in zone 1, inside your RER ticket's covered travel zone (from CDG, zone 5, to central Paris, zone 1). If you instead continued on RER B down to say Bagneux (zone 3), using only the standard CDG to Paris RER B ticket, you wouldn't be able to exit the RER station. The fare gates at RER exits require you to insert your RER ticket to check for appropriate zone coverage. The RER gate will not open if you're outside your ticket's coverage.
Metro exit gates/doors/turnstiles have no ticket slots as the Metro system doesn't need to check for proper coverage. Paris Metro is all one zone. This is why you can make your RER to Metro transfer and exit at Babara (RER zone 3, but it's Metro, so zones don't apply) without needing another Metro ticket. If you wanted to make the trip only on RER B from CDG to Bagneux (somewhat close-ish to Barbara Metro station) you would have to pay extra to get the coverage from Zone 5 to 1 to 3. (Just checked on Transilien.com, it's 14.15€ for the RER from CDG to Bagneux.)
To confirm what you see I used the Bonjour RATP app (I assume that's what you meant by "app") and see the same price: 11.80€ / single fare. So, just to confirm again, you'll need only your 11.80€ RER B ticket to Paris to complete your journey to Barbara. (Note: at Denfert-Rochereau, you need to stay inside the station and make your transfer to Metro 4. Don't go up to street level and wander about and try to return for transfer, this is not covered.)
That was a good question. It's a confusing/complex situation with the mix of RER, its zones, and Metro, without zones.