Mormon now returns to the story of Alma that he left in our chapter 18. His concluding statement of that part of Alma's story is:
Mosiah 18:34
34 And it came to pass that Alma and the people of the Lord were apprised of the coming of the king's army; therefore they took their tents and their families and departed into the wilderness.
Mosiah 18:35
35 And they were in number about four hundred and fifty souls.
Textual: An inserted notation about the following chapter precedes this verse. The sentence: "An account of Alma and the people of the Lord, who were driven into the wilderness by the people of King Noah" is probably from the text on the plates, and is a direct structural parallel to the preface before our Mosiah chapter 9 that introduces the record of Zeniff.
Mormon's inclusion of these introductory phrases tells us that he is beginning a major divergence in his current story line. In the case of Zeniff, it also certainly denoted a change in the source material for his text, which is taken from the plates of Zeniff.
Here too there is a clear change in source material. He has been giving the story of Zeniff through Limhi, and now proceeds to give us the story of Alma the Elder. While this material certainly had to have some source in Alma's records, this is not a clear designation for the material Mormon includes. In the 1830 edition of the Book of Mormon, this introduction serves to present chapter 11. That single chapter is now represented by Mosiah 23-27 inclusive. At some point has been added the text that indicates that this introduction covers only our chapters 23 and 24, but that appears to be a misreading of Mormon's intent.
If we presume that chapter breaks are Mormon's own divisions in his text (as they appear to be) then Mormon is considering our current chapters 23-27 as an entire unit. This unit is not simply the story of Alma before he arrived in Zarahemla, but the story of Alma the Elder, including his importance to Zarahemla. Alma the Elder is a pivotal character in the Book of Mormon, notwithstanding he has no "book" named for him (as does his son, Alma).
The two included summaries give us a little more information about the way Mormon was organizing his records. Both the record of Zeniff and the record of Alma the Elder are clearly separate plate traditions from the large plates of Nephi, yet Mormon includes both of those records under the "book" heading from the large plates of Nephi (which is the Book of Mosiah, named for Mosiah 1).
Mormon's conceptual pattern appears to be following the overall structural lead of the large plates of Nephi, and filling in important pieces with supplemental records such as those of Zeniff and Alma the Elder. Nevertheless, he considers his story to be that of the Nephites as defined as that political unit that kept the plates of Nephi (large plates).