Train types, Rouen to Paris

There won't be meaningful differences between trains for this short ride. However, Rouen-Paris is an exception among French regional lines, in which advance reservations are cheaper (if you are able to commit to a specific train). So, it pays to book that one in advance.

Trainline is probably seeing different ticket types and making guesses as to what they mean. That is what you get when you add a layer between you and the train company...

Have a look at www.sncf-connect.fr

There you will see what the actual train types and fares are called. The trains to Normandy are TER trains, and the faster ones operate under the "NOMAD" brand. They are all operated by SNCF.
Trains to/from Paris require a reservation, and you can get discounts when you book in advance. So maybe that is what is confusing trainline.

SNCF actually runs the trains and is the only place I ever use to book tickets. Booking with anybody else is just using a middleman.

Thanks everyone for your comments about the train. I'm aware that I am using a "middleman" but that's because I found the SCNF a little confusing, so I'm willing to pay a small amount for overall greater clarity.

And indeed, booking in advance was very cost-effective!

Merci.