At this point, it had become clear that the Nephites were not going to be able to hold on to many of their lands. Mormon understands that the Nephite archive in the hill Shim was threatened. Mormon had been to the hill, as commanded, and had removed the Nephite record he was writing on. That single collection of plates was not the whole of the archive. It could not have been. There had been nearly a thousand years of Nephite history, and they had to have been written on many sets of plates.
Words of Mormon 1:3 describes Mormon searching through records to find the rest of the reign of Benjamin, only to find a set of plates (which we know as the small plates) which he had not known were there. If there had only been a couple sets of plates, that explanation would make no sense. There was a large archive of records. They were under threat. Mormon retrieves all of them from the hill Shim.
This event is critical for understanding when and how Mormon created the Book of Mormon. It was an abridgment of all of Nephite history, and history that wasn’t even available to him until after he removed these records from the hill Shim. Therefore, the conception to write the Book of Mormon, and the source materials used to write it, were not available before this time, sometime after the defeat at Boaz.
This ends a chapter in the 1830 edition of the Book of Mormon.