The Citystar-ticket is only available for Russian destinations. It makes sense to use the Citystar-ticket for long domestic trips in Russia or trips from Europe to Russia or back.
If you travel to/from China/Mongolia the Citystar-ticket can only be used for the European and Russian part of the trip.
You can for example use the Citystar-ticket for travelling via Slovakia to Irkutsk and then use another international ticket from Irkutsk to Ulan-Bator.
If your trip starts in Moscow, it makes usually no sense to use a Citzstar-ticket for the trip Moscow - Irkutsk. Other options are usually cheaper.
European railways don't sell international tickets for trips to China or Mongolia. You have to get this tickets in Russia (or Mongolia or China) or with the help of a travel agency.
I'm in contact with an agency in Moscow, they can book also this tickets for you.