Thank you for your request.
The Swisspass is only the carrier medium. The decisive factor is the service, the subscription on the SwissPass.
I assume that you have the Swiss Travel Pass. See in the map where it is valid. For a trip from/to Milano you need a ticket, as the route is not included (see map of validity). Likewise, in the EC train a seat reservation (for abroad) is mandatory.
If you have any questions, we are here for you. Have a nice day.