Words of Mormon is the shortest and most structurally unusual book in the Book of Mormon: a single chapter written not by any of the small plates authors but by Mormon himself, the prophet historian who compiled and abridged the larger Nephite record centuries after the events of Omni. Mormon explains that he has finished his abridgment of the large plates down through the reign of King Benjamin and has, in the course of that work, come across the small plates of Nephi, an older and separate record he had not originally planned to include.
Rather than leave the small plates aside, Mormon states that he includes them for a purpose he says he does not yet fully understand, though he trusts it to be a wise one, a line that later Latter-day Saint tradition reads as a moment of prophetic foresight given how the small plates would later fill the exact narrative gap left when the original opening pages of Mosiah, described elsewhere as lost, dropped out of the manuscript that reached print. Words of Mormon is thus a hinge point, marking the transition from one authorial voice and one physical set of plates to another within a single continuous book.
Latter-day Saint discussion of the Book of Mormon's composition treats Words of Mormon as an important piece of internal evidence for how the finished volume was assembled: it is the one place the text openly names its own editorial seams rather than presenting the whole compilation as a single seamless narrative voice throughout.