Geographic: Mormon gives us a picture of the holdings of the land of Zarahemla at the beginning of this next round of conflict. Moroni has cleaned up the boundaries by strengthening the eastern boundary and therefore lessening the opportunity of the Lamanites to have an easy entrance deep into Nephite lands (the last attack made it all the way to Jershon).
This verse tells us that the line was roughly west-east. The Nephites solidified that front to have strongholds of defense along the most likely entrance points. We also learn that the southern line of the land of Zarahemla extended south to about the location of the “head of the river Sidon” which appears to be the headwaters.
The land northward is interesting because Mormon indicates that they have possession of all the land northward, even north of Bountiful, which appears to be the northernmost land of their possession at this time. It would appear to mean that the Nephites did not have any rivals to the north, and therefore considered that land theirs because it was available to them rather than because it was already under their control.