Hello Interrailers and Eurailer!
How you (almost) all know, in Germany and Austria you don’t have to book a reservation, but you can via bahn.com (4€/trip 2nd class) and tickets.oebb.at (3€/train) via seat only option.
Some trains are now and I presume this summer very full. Please check before boarding via bahn.com the utilization forecast for German trains and via fahrplan.oebb.at the utilization forecast note for Austrian trains. For trains that will be full you get a note with the text that a reservation is preferable to get a seat. Some trains now, have a note that due to that this trains are full, you only are allowed to get in the train with reservations. Without you have to leave the train.