Eurostar always needs reservations.

The IC Direct service Brussels - Amsterdam is not reservable, just add the train to your pass, board and pick any seat.

Apologies if these are stupid questions but is this included in the Eurail pass or do we need to pay for a further ticket ? We already have the Eurostar leg booked.

Ok I have added this trip to the rail planner- thanks for that advice. I’m just nervous not knowing that we have definite reservations.

The intercity train from Brussels to Amsterdam is included in the pass and you don't need any reservation or supplement for it. The app recently says there's a supplement but that is not correct.

There are also faster Thalys high-speed trains and for these you need a reservation.

But these Thalyses have quite high reservation fees by the way, limited places for passholders, limited flexibility and not that much quicker between Brussels and the NL. Usually it’s always a better deal to go for the IC with a pass. It also gives you the chance to discover lesser known cities, like Mechelen (Be) or Breda (Nl).

Just to get the terminology correct. You can have only one Trip connected to your pass. To this Trip you connect Journeys, which are the different trains that you will use. Just before boarding a train you activate the journey for that train to get the QR code (your ticket).