In 3 Nephi 5:9, Mormon indicated that he would use a “shorter but true account” for this book. It appears that the heavy quotation from that account might have begun in 3 Nephi 8. Here, in 3 Nephi 11, Mormon gives us a chapter header. Mormon has used those to indicate a change in sources, but that might not be the case for this chapter. Certainly, he used Nephi’s record for this account, but he might have begun using that account much earlier.
What is certain is that this is a significant chapter, and a significant part of Mormon’s message. Therefore, Mormon introduces this most important part of his work by declaring that it is the witness to what happened with Jesus Christ, or, in Nephite terms, Jehovah on earth, when he appeared to the people gathered in Bountiful.