Optional reservation: you can book a seat but it is not mandatory. You can board the train and find a free seat. Reserved seats will normally be indicated on the seat, the window or the overhead luggage rack, by mentioning the section that has been reserved. Without a reservation, you are never 100% sure to have a seat but if you're alone or with just 2, chances are usually high there are free seats. Fridays, Sundays and days around public holidays are often busier of course.

For ticket checks: your Interrail pass is your ticket.

When tickets are checked, you always need to show your Interrail pass. On a paper pass you need to write the train into the travel diary and (for a flexible pass) fill in a travel day on the pass. For a mobile pass, you need to add a train to your trip and to your pass and then a barcode ("ticket”) will be generated to show to the staff. If you have a mobile flexible pass, you will be prompted to activate a travel day if necessary.