Pass and reservations are separate and don't need to be linked.

Whether you can book the reservations via ÖBB depends on the trains involved. If you share your timetable, then people can advise. Via ÖBB you can indeed book for multiple people in one go.

(Note: saving journeys in the app is not booking, it's just the electronic equivalent of writing the trains on the paper pass trip report. It is not needed until you board a train.)

Thankyou so much for your quick reply!! This is my timetable: 

Also, I'm a bit confused.. So I cannot book the train through the Rail Planner? Do I book it through ÖBB as well? And when I do, have I booked the train as well as the seat reservation? When I try to book this through ÖBB it says ‘Ticket valid for section of the journey only’. What does that mean?

For your first leg Utrecht-Zürich I would recommend to buy the reservation (it is only optional on this trains) via bahn.com with the seat only option (4,50€ for the hole journey/person from Utrecht to Zurich and you can choose your seat on a seat plan). 
From Zurich to Milan you can use via ÖBB like in the screenshot. You can reserve the trains also via ÖBB with the seat only option (only for the first trains with optional reservation) it costs 3€/train/seat, no seat plan. 

Thankyou!! But how do I book the train journey? Is booking the seat reservation the same thing as booking the train?

Yes you go to bahn.com and choose seat only option. 

The rail planner app does not book trains. It just forwards to some of the websites that can book them.

The trains you show until Zürich all have optional reservations but you can book them for just €4.50 per person and journey via DB (use "Seat only”). The remaining train to Milan can be booked via ÖBB indeed for 11€. You can also change to a regional train in Chiasso and save 11€. Seat reservations within Switzerland are not necessary and generally a waste of money (unless part of a journey booked via DB, since they charge optional reservations per journey and not per train).

I see you have already activated the pass and a travel day. That is not necessary. Moreover, it is not a good idea to do that in advance, since you can loose a travel day if you change plans and forget to cancel your travel day. You can only cancel a travel day until 23:59 CET on the day before it starts.

Finally, please don't use the rail planner app to plan. It is not reliable enough for that purpose, since it is only updated about once per month. National planners or apps are more reliable (e.g. www.trenitalia.com for Italy). For international planning, the DB website or DB Navigator app are a good choice (updated twice/week).

Thanks so much!! I have one last question. I can book the trains until Zürich, but if I want to book the train from Zürich to Milan, it says: ‘reservation impossible”.On DB as well as ÖBB. Does this mean there's no seats available? 

Online, DB can't book reservations only for this train (at least not into Italy).

ÖBB can but do NOT use the “seat only” option for this train. Instead, add the "Interrail / Eurail” discount to each passenger and then look for normal tickets. Also see these steps.

You have to book the reservations to Zurich with bahn.com or oebb.at (seat only option) and then Zurich-Milan via ÖBB (one way ticket option)

@rvdborgt @Angelo I finally managed to book the trains!! Thankyou both so much! :)