1998, Robert Z. Apte, Andrés R. Edwards, “The Geography of the Land of Snows”, in Tibet: Enduring Spirit, Exploited Land, 1st edition, Heartsfire Books, →ISBN, →LCCN, →OCLC, page 20:
The Northern Plain (U-Tsang) The Northern Plain is the largest region of the country. This vast area called the Changthang, covers a large portion of the country. To the extreme north is a cold, desolate desert quarter neighboring on the Chinese-Turkestan border that is mostly uninhabited. The western part of this region, which borders on Ladakh, supports a small number of nomads who live off the scant vegetation. In the 1950s, U-Tsang was sectioned off by the Chinese and named the Tibetan Autonomous Region (TAR) (See photo, pg. 37).
[2008 March 21, “Who are the Tibetans, what is Tibet?”, in Reuters, archived from the original on 27 June 2022, Global Coverage 2:
The government-in-exile calls Tibet “Cholka-sum”, meaning “The Three Provinces”. These historic areas are U-Tsang, which roughly corresponds to the TAR, Amdo centred around Qinghai Province, and Kham centred around Sichuan in China’s southwest.]
2020 July 11, Akhilesh Pillalamarri, “History of Tibet-Ladakh Relations and Their Modern Implications”, in The Diplomat, archived from the original on 13 July 2020:
The Tibetan plateau — the geographic and cultural region associated with Tibet — has traditionally been divided into four historical regions. Three are almost entirely in China: Amdo in the north, now associated mostly with Qinghai and Gansu provinces in China, Kham in the east, split between Sichuan province and TAR, and Ü-Tsang, or central Tibet, the region is generally identified with the idea of Tibet, both culturally and administratively, although parts of Ü-Tsang extend to northern Nepal and the Indian states of Sikkim and Arunachal Pradesh.