Mormon tells us that his source is the “record of Alma.” This is the same record that we have previously seen for the sermons in Zarahemla, Gideon, and Ammonihah. The major difference is that this will be a story of the sons of Mosiah, not Alma. As this chapter begins, we are introduced to the way that the story of the sons of Mosiah ends up on Alma’s personal records. Alma meets them, and they tell their story, or Alma receives records from them that will tell the story.
When Mormon uses that separate source, he provides a chapter header to notify his readers that he is taking text from a source that was not on the large plates.