When you book all seats in one go, you'll be seated together if possible.

The best website to book depends on the train. For Austria and Italy, it's usually tickets.oebb.at. Cheaper and more reliable than the Eurail website.

As to Salzburg to Florence: what's your travel date?

The first thing to understand is that the Eurail app does not reserve seats at all. It will forward to other sources which may or may not be the best options for reservations.

The Eurail website reservation portal is the usual place you get forwarded to but it is often problematic and does not have the ability to choose seats on any trains.

With a few exceptions the only place where you can select specific seats is on the website of the train operator of each train, even then this is not universally available.

When booking multiple seats in one transaction on most platforms it is expected that those seats will be close but there is no guarantees unless you are able to select specific seats. Be careful with preference options as if you select “window” or “aisle” for 4 reservations then they by definition will not be together.

Please read these threads which details many places to get reservations.

Thanks, I have been going to the Obb site for this, we were looking at doing nightjet on 24th Sept at 22.00ish but can't seem to get tickets, found train that goes to Vienna at 16.11 then to Florence but is only seated overnight. We are also looking at train from Paris to Munich on 15th around 9am so where do u think best for tickets app or other. Our other trips are just day trips in Germany and Italy and Munich to Salzburg but they seem pretty straight forward 

There are still couchettes available on Vienna-Florence. Best to book is nightjet.com. Add the Interrail/Eurail discount for each passenger. Also best to take the 16:07 from Salzburg. It arrives a lot earlier and a small delay won't then break the connection.

That would be Paris dep. 9:06, with a change in Mannheim.

For Paris-Mannheim, book online via Eurail (+€2 booking fee per person) or call SNCF (#85 for English, no booking fee, delivery via email).

For Mannheim-Munich, book on tickets.oebb.at (€3 per person in 2nd, free of charge in 1st class) or on bahn.com with seat selection (€4.90 in 2nd or €5.90 in 1st per person).

Thanks again, are you saying get 16.07 train to Vienna separately then Vienna to Florence at 19.18 but can only see seats not couchettes, app says reservation will be made, as doing to as a single trip from Salzburg via Vienna doesn't give this option 

Here is some useful information from the experienced travellers in the Community regarding both planning, reservations and activation of pass and travel days. 

 Planning

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

 Reservations