Booking train tickets with a swiss pass

I am travelling from Milan to Luzern
My train is Eurocity EC 12 train timing is as follows
Milan 7.10>Chiasso 7.49
Chiasso 7.52>Arth goladu 10.09
Change with IR 2170
arth goladu 10.14>luzern 10.41
My question is if i have a Swiss pass & i book my ticket from Trenitalia from Milan to Chiasso can i remain on the same seat for the Chiasso to Arth goladu part without booking for the latter part
Thanking you awaiting your reply

Hi Div. You'll have to make seat reservations for the entire leg up to Arth Goldau if you want to remain in the same seat. The Swiss timetable allows you to book a ticket including seat reservation for the entire leg, and including the free trip up from the Swiss border with your Swiss Pass. In order to do so, enter your trip, choose your connection, click 'Fare/Buy' and enter the order procedure. When asked for a reduction option, select 'GA' (this equals the Swiss Pass in terms of free taveling and discounts) in either 1st or 2nd class.