Please be notified that the Chinese Visa and Tibet Permit are two things entirely separate. Four documents are required if you want to travel to Tibet freely.
Chinese Visa- It can be applied for at the Chinese Embassy in your country.
Tibet Tourism Bureau (TTB) issued the Tibet Entry Permit and is compulsory for all foreign tourists entering Tibet.
The Alien Travel Permit is issued by the PSB and is required when you plan to travel to the remote areas of Tibet. Your local travel company will obtain it after you arrive in Tibet.
Military Permit – your travel company will have to obtain travelling to militarily sensitive areas.
Travel to Tibet from Nepal
If you enter Tibet from Kathmandu, Nepal, you must obtain a Chinese visa at the People’s Republic of China’s consular department in Kathmandu. An outsourced agency does it, and your Nepal travel company will handle this for you. The visa processing for Tibet is only open in Kathmandu between 10 am to 1 pm from Monday to Friday. Please note that this visa application is compulsory, even if you already have a Chinese visa from your country. The border treaty signed between Nepal and China has regulated this.