Which reform provided the most direct benefit to Darius I's bureaucracy in the Persian Empire?

Darius, who ruled from 521-486 BCE. Darius used military might to expand the Persian Empire into northwest India to the Indus River, and in the process conquered Thrace, Macedonia, and the western coast of the Black Sea. Under Darius' rule, the population reached 35 million, which was the largest empire in the world at that time. To keep control over such a large empire, Darius instituted a centralized the government, and built a new capital at Persepolis. Darius was an important ruler because under his rule the Persian empire became the largest in the world, and also because Persepolis served as the center of the Persian Empire even after Darius' decline. After Darius came Xerxes, who ruled from 486-465 BCE. Unlike Darius, Xerxes was not a tolerant ruler, and he imposed his policy on the lands he had conquered. This caused much unrest, particularly in Mesopotamia and Egypt. Despite the unrest Xerxes managed to suppress rebels and maintain order, but not forever. Xerxes was important because the unrest caused by his intolerance brought about problems for emperors later in the Achaemenid Empire.