The province at Persia’s western flank was known as Beyond the River.
The river alluded to here is the Euphrates. This province had multiple governors over its multiple districts. Lest they be hostile to Nehemiah as he passes through to his destination (which was to be expected), he seeks safe conduct from the king by way of written notice.
7 And I said to the king, “If it pleases the king, let letters be given me to the governors of the province Beyond the River, that they may let me pass through until I come to Judah,