This is the first time we have seen Mormon’s record have a chapter header. There are two probable chapter headers in 1 Nephi that were not typeset that way, one before 1 Nephi 1, and the second before 2 Nephi 6. John Gilbert, the compositor for the first edition of the Book of Mormon, and the one who created the sentences and paragraphs, recognized this one as a header and set it off. He was clearly correct.
Mormon announces that he is taking his account from the record of Zeniff. As we saw in the comments on Mosiah 8:5, the record of Zeniff appears to cover the records of Zeniff, Noah, and Limhi. Mormon only notes that he is beginning with the record of Zeniff.
As is typical for Mormon, he announces when he takes material from a non-large plates source. However, he does not similarly mark the return to the large plates as a source. The book of Mosiah will be an excellent demonstration of how Mormon handles his movements between his standard large plates source and the secondary sources he uses as supplements.