The other set of plates, the ones we know as the large plates of Nephi, were created first. Upon those, Nephi began writing “an account of the reign of the kings.” Those plates are an unbroken tradition to the last Nephite archivist, Mormon. They would form Mormon’s foundational source for his masterwork.
This set of plates, which we call the small plates of Nephi, had a different purpose, and a different line of transmission. Where the large plates followed the ruling line, the small plates were given to Jacob, and then to his son, and son’s son, and so on. One of the unstated, but significant, differences between the sets of plates was that the rulers could command and create new plates. Apparently, Jacob’s descendants could not. They wrote on the blank plates Nephi had originally created, and when there were no more, the record ended (see Omni 1:30).
Nephi says that these plates are for the more part of the ministry, yet they are heavily historical. Nephi does not intend that these plates contain only sermons. The way they are written, they justify the ministry. They establish Nephi’s promise from Jehovah, and demonstrate its fulfillment.
There is an original chapter break at this point, but not one that was in Nephi’s original plan. Beginning with Nephi, and continuing through Mormon’s writings, when amen appears in its function of testifying, it causes a chapter to end. This doesn’t occur if it follows a recorded prayer for instance, only when it is intended to testify to the sermon, or what was written (as in this case).