Isaiah 16:13

This is the word that the LORD hath spoken concerning Moab since that time.

This is the word about Moab spoken by the Lord, a prophecy belonging to the series of burdens concerning the nations, including Moab, in chapters 13 to 23. That prophecy is directed against Moab as one of the oppressors of Judah. It stands within chapters 13 to 23, the part of the prophecy directed against foreign nations.

It therefore belongs to the first part of Isaiah, chapters 1 to 39. That first part consists of single prophecies and historical accounts from Isaiah's active public ministry as God's messenger to the rulers and the people. They were first published singly at different times and on different occasions and afterward gathered into one collection while still remaining distinct and single.