1st class obviously and then all the seats are the same. I don't see the difference between 1 and 2 (don't think there is one on the Giruno train you'll take). 3 is for the quiet coach which you don't really need or want. 4 is the same as 1 (there's no priority boarding whatsoever). Don't know why there are so many options, it's a bit confusing...

Then choose 1 or 2.

Please note that it's better to use tickets.oebb.at for connections in and to Italy. There's no booking fee (compared to 8 x 2€ on eurail.com) but you'll have to split the booking.

Add Interrail/Eurail as a discount and select one-way tickets.

Did you see the answers for the journey between London and Paris ?

Hi thibcabe, may I pls understand why “select one-way tickets” and not “seat reservation only”? 
 

yes I saw your answers and found my way for London-Lille Europe, then Lille Europe to Paris Nord on 27 May, thank you for your help

Because that's how it works on tickets.oebb.at

You get a "Eurail discounted" ticket aka a seat reservation.

Thx so much thibcabe! Got my tickets

make the reservation in a SBB ticket center (train station) for the first class, and make sure that all seats are together at the same place. they can do that for you and you can pay directly on site