You need to show both, You need the valid travel day on the Mobile Pass and show the Reservation code or pdf upon inspection. For night trains, you best print it out. As train staff usually keeps the ticket with them.

There’s no way to link the two, reservation is something seperate than your ticket (=QR-code on the app).

You always need to activate the ticket before stepping on the train, a couple of minutes before boarding is just fine. It’s quickly done :)

“Also I see that the train from Bruges to Amsterdam requires a Supplement, but it says is Included for International Travel, so in theory I dont need to worry about it?”

Indeed, that’s an annoying bug that shouldn’t be there. International ticket holders (so also pass holders), don’t need to buy the supplement. The supplement is only for people with domestic ticket. No worries.

The reservations are completely separate from the pass - there’s no way to view or add them to the app. So you will always have to show two things to the inspector:

• the pass, with correct trains added, and a QR code to show
• the reservation - usually on your phone is fine.

I will let somebody else explain about the supplement, but I believe you don’t need to pay any extra with. a pass.