SAVER Hello.
You can use a Saver Day Pass only in Switzerland, from the border station Domodossola to Zermatt and from Zermatt to Domodossola. If you go back to Milan on the same day, only one day pass is needed. You have to buy these passes in advance on sbb.ch to get the best prices, they are not refundable.
From Milan to Domodossola and from Domodossola to Milan you have to buy tickets. Seat reservations are mandatory in the Eurocity trains (EC) and will be delivered with the tickets. You can buy them in advance on sbb.ch, trenitalia.com or at the ticket office in Milan.