Reservations ?

In Germany, and other countries, I think, regional trains do not have reservable seats. You don't need a reservation to ride them. In other places, where a time and route are popular, you might want to reserve a seat, but a reservation is not required. However, without a reservation, you might have to stand. In Germany, a few trains, that are expected to be crowded might suggest a seat reservation. For these, reservations are usually around €5. Lastly, a few high speed trains are considered "Premium" trains for which a higher fare is charged. If you want to ride one of these train with a rail pass, you have to pay a surcharge. Passholder tickets for these train come with a seat reservation, so the surcharge is often mistakenly referred to as a reservation.

It varies by country and train system. Generally the slowest trains are non reservations. And all premium or high speed trains require reservations. In between will depend on the train system. Some will be a combination of reserved and non-reserved. And other will be all reserved. In Italy only the regional trains are non reservation. The InterCity or IC are required reservations. In Germany and Switzerland I think these trains are a mix. Just look on a schedule and it will be noted, generally with a small r if a reservation is required.

Do not use EUraide for reservations as Rick suggests. They charged my credit card for railpasses and reservations 20 days ago and they still are not shipped. Reservations have to come from Munich and have not shipped yet. Now we leave in just over 1 week and I do not expect them to come before we leave. It has been 6 weeks since I first contacted them. I could have ordered Eurail passes on my own and had them weeks ago. I am sure I could have figured out the reservations on my own by now or got them on our arrival in Europe. Now I have to decide if I should try to reverse the charges on my credit card and re-order.
I don't have time probably to do that either, so I am probably out $1700.00.