That is entirely correct. In top of that, bahn.com is cheaper than Eurail.

If it helps I think in terms of low cost airlines. You buy a ticket to fly (your pass) and you have to pay a little extra for a chosen seat (your reservation). These are effectively mandatory reservations. Like airlines, trains requiring reservations do not allow more passengers than seats so no standing allowed.

Several years ago British Airways actually operated regional flights with a guaranteed turn up and go option. (They actually had a back-up plane that was available if the first one was full that would depart 10 minutes later). This is similar to optional reservations where when seats run out you can still travel but may need to stand.

For trains without reservation you simply add details to your pass before boarding and travel.

For trains with optional reservations you can either take the chance of finding a seat or standing or pre-purchase a guaranteed seat. You then only use the reservation to eject errant passengers from your chosen seat.

For trains with mandatory reservations you have to pre-purchase your guaranteed seat and present it in addition to your pass with the details of the train added. Reservations are always a separate item to your pass, even when bought through Interrail.

Except that for THESE trains-in both 2nd and 1st, there is no limit nor any quota nor even any real need to REServe. The cost for 1 1st cl seat booked as ´seat only´ via bahn should be around 6€. You could also try via Oebb.at-no idea if they offer such fancies as choosetheseat- their pirces are often a little lower-but at such levels unlikely to be of much interest to the 1st cl. clients

The need to book a seat depends very much on the day and time of travel. As you well know, it can be very busy on some days, especially between Prague and Berlin.

Thank you very much!!! Now I feel confident in booking my reservations.