Tibet is defined first by altitude. The plateau sits mostly above four thousand metres, walled on the south by the Himalaya and on the north and west by further ranges, and the air, the growing season and the routes across it all follow from that. Barley that ripens fast enough to matter, yak that can work at that height, and salt from the northern lakes are the materials of the traditional economy, and the valleys of the Yarlung Tsangpo in the south carry most of the farming and most of the population.
Height did not mean isolation. It meant that movement was funnelled, and whoever held the passes held the trade. The Tibetan empire of the seventh to the ninth centuries took the Silk Road oases, fought the Tang for control of the corridor and for a period held its capital, and it was during that empire that Buddhism arrived with royal sponsorship, that a script was devised for the language, and that the state began commissioning translation.
When the empire collapsed in the ninth century the state disappeared and the religion did not. What followed instead was a rebuilding from the monasteries outward: teaching lineages founded by individual masters, monastic estates that accumulated land and tenants, and eventually institutions strong enough to govern. From the seventeenth century the Ganden Phodrang, the government of the Dalai Lamas, ruled central Tibet as a religious administration.
The atlas holds Tibet as a region in its own right because the system it produced, its canon, its pilgrimage geography and its monastic network, crosses several modern state borders and belongs entirely to none of them.