Hi @sdk
You will receive by mail or at the train station - if you bought the transport at the train station -, a green plastic cover, the transport label and the customs declaration for the return journey from abroad contained in it. You must fill in the customs declaration before your return journey, or before check-in at the airport abroad, and attach it to your luggage with the green plastic sleeve.
You can also find this information on Flight luggage: transport and check-in at Zurich Airport | SBB
I wish you a pleasant day.
Greetings
DafineM