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This pass is valid only in Switzerland, you need a ticket from Domodossola or from Chiasso to Milan. If you travel with an Eurocity, seat reservations are mandatory in Italy and are included in the ticket.
If you travel via Chiasso, you can take an IC 21 from Basel to Lugano and change to the RE80 from Lugano to Milano. You can buy the ticket from Chiasso to Milano on sbb.ch or in SBB Mobile app, it is called Arcobaleno Transfrontaliera and it is valid only in regional trains. It is not necessary to buy such tickets in advance. Be sure to select the right train from Chiasso, the ticket is valid only for 4 hours.