Why not mentioning Tibet travel when applying China visa
It is because Chinese embassy requires Tibet Travel Permit when you are applying China visa, but the case here in Tibet is the government will not issue Tibet travel permit unless you provide passport + visa copies, so it is a contradictory policy, and very stupid actually. That is why travel agents suggest you not to mention your Tibet tour to the embassy, you can just tell the embassy that you are visiting China mainland. Visitor do not follow this advice will 100% be rejected.
What documents to show the embassy
You might (or might not) need an invitation letter from travel agent to prove that you have already book a China tour with a local Chinese agent, once the embassy sees this invitation letter, they will let you go and issue the visa in a few days. So your travel agent can make an invitation letter for you, in the invitation letter your Tibet tour will be replaced by a China tour, so that you can use it to show the embassy and pass the application.
Invitation Letter from our agent
For making the invitation letter, the travel agent needs your China arrival date + arrival city name, China departure date + departure city name, once we have these info, a invitation letter will be worked out.
The embassy will NOT need to check your China domestic flight/train booking as they know those domestic flights or trains are usually arranged by local travel agents, while international flights are usually arranged by travelers. So embassy will see your international flight booking only.
There is no direct flight from abroad to Tibet, so you probably will fly like this route: Home-China-Tibet-China-Home, which means you will book the international flights for Home-China & China-Home, these 2 section tickets are what the embassy wants to see, just show the embassy with these international flight booking paper, and bring the invitation letter issued by us , then you can get the visa.
Last but not least----If you are doing Kathmandu-Tibet route, you will NOT need any China visa to be applied at home, instead you need a so called Tibet Group Visa, which is also called TGV, which allows you to travel from Kathmandu to Tibet and mainland China. For more info of this Tibet group visa, you can check our article: www.travelchinatibet.com/tibet-permit-visa/tibet-group-visa.html
See a sample of invitation letter for China visa application