Depends on which countries you want to visit and which trains you'lld like to take. 

The pass= ticket. Seat reservations are separate from the pass, and app and require an extra fee per seat (although you can minimise it in a lot of case, even bypass them If you have a lot of time). 

State your travel plan, and we can help. 

Spain, France (and a little less Italy) are quite reservation heavy countries. 

In a nutshell, there are

• Reservation free trains (Most trains in Europe, mostly local and regional services. S, L, IR, RE, RB, Sprinter, R, TER,… many different names)
• Trains with optional reservations (Long distance trains in Germany, Austria, and the UK; IC, EC, ICE, RJ,...). You can board any train you’lld like and find a free seat, but if someone shows up with a reservation you’ll need to find another seat. Advisable to reserve on busy travel days, like summer-weekends, long holiday-weekends, or when travelling in a small group…
• Trains with mandatory seat reservations (Nighttrains, TGV in France, Eurostar, Thalys, Frecce-trains in Italy, AVE's in Spain). Usuallyhigh speed, long distance (sometimes International) trains. Without a seat reservation, you don’t get on the train. These trains can sell out, sometimes weeks in advance (Eurostar). 

If you let the Community know your travel plans you will get advice which reservations you must make and what reservations that might be good to make.