Verses 6–13 are an insertion into the text. In these verses, Mormon the narrator is replaced by Mormon the editor, the one who selected the texts to be recorded. He notes that it is impossible to write everything that happened on his plates, but there is a more complete account on the plates of Nephi. When Mormon uses the term plates of Nephi, he is referring to what we call the large plates of Nephi. He doesn’t mention the small plates of Nephi except in Words of Mormon, where he specifically introduces them.
Mormon notes that he has had to make selections from the material available to him, and reiterates that his guiding principle in making those selections was “to the intent that they may be brought again unto this people, from the Gentiles.” The statement that they would come from the Gentiles indicates that he knew, as he wrote, something of how the Book of Mormon would come forth in the latter days.
Verses 9 and 10 speak to those future descendants of the New World house of Israel who would receive the Book of Mormon.