Salt and tea. It is a necessity for ancient people, especially in the era of limited resource capacity at that time, which is particularly important and needs production. In ancient times, salt was basically produced by government-run saltworks and salt households designated by the government. When salt is produced, it is naturally an important material for economic strategy, and it is transported from centralized production sites to all parts of the country. Tea has always been an important commodity for trade with northern nomads. Due to geographical reasons, the nomadic people in the north basically have no tea resources and are trading.