Geography: This passage defines a difference between the land of Nephi per se and the areas where Lamanites lived. The land of Nephi ran from the east sea to the west, meaning that it was under the control of one leader and was seen as a regional unit. It also had an extensive frontier. This passage also explains that Lamanites lived north of the land of Nephi proper. These inhabitants would not be officially part of the Lamanite hegemony (most likely there were several), probably because they did not have a major city.
Moroni’s removal of the Lamanites from the eastern territories would not have helped much unless he settled Nephites in that territory, though the text does not indicate that he did so. Probably he intended to match the Lamanite control of a border with a similar Nephite line.