Hi!
RJ66 is not a regional train. RJ stands for Railjet…
So or so, for this train NO reservation is needed for international trips.
The information in the app is wrong. Only if you’d travel Budapest-Györ (e.g.) you’d need a reservation as for domestic InterCity trips in Hungary a reservation is compulsory.
This is NOT a regional train, the R means RAIL=Jet. There are NO Reg trains all the way here-these start from Györ.
But for trips going OUT of HU RES is optional-and it is not for domestic, strange but true. Seats are marked as being RES and for what sector in the train. You can assure yourself also of a seat by just buying the local MAV/HU seat till some stop in HU-RES on a yellow machine for 3/400 HUF=less as 1€-that you sit anyway.
Sorry, my bad, I thought R stood for regional! Thanks a lot to both of you, it has been of great help :)