What train is it? Date, departire time and route?

@Martal as I understand it, seat reservations are only compulsory on some international DB trains such as the ICE to Denmark or Paris:

https://www.interrail.eu/en/plan-your-trip/tips-and-tricks/trains-europe/high-speed-trains/ice

I don’t think the ones to Switzerland, Austria and the Netherlands are compulsory, e.g:

https://support-en.nsinternational.com/app/answers/detail/a_id/15032/~/ice-%C2%BB-facilities-on-board

Judging by the people I’ve seen sitting on suitcases at the ends of ICE carriages I think a lot of people just turn up with a ticket.

This is also useful:

https://www.seat61.com/trains-and-routes/ice.htm

It’s the ECB 88 from Brenner to Munich 31/07 at 12:00 am

THIS trains are so often explained here: the RES as such is not mandatory (must do) but there is  special SURcharge of 10€ for all trips on these trains into/out of Italy. If Brennero also counts to this-I dk, but it is usually thought of as the tariff border separation point between AT and IT, even though it is in IT, so it should not be. The app cannot post this clearly and thus tells you its a RES. As such a RES is often very useful in hi-peak summertime. There are also hourly locale OeBB treni to Innsbruck and these are certainly without.

Also note that to reach Brennero dalle IT (Bolzano) you have to use local/regionale treni dalle SAD and these do NOT accept passes.

No ICE need to be RES-except those running in coöperation to FRance with SNCF. The reason why you see now so many people standing/lying down etc in the far to populair AMS trains is hi-summer-peak, NO need to RES and the cancellation of many of these trains=make the next one double busy.