Alma 3:11 - 3:15

Alma 3:11 And it came to pass that whosoever would not believe in the tradition of the Lamanites, but believed those records which were brought out of the land of Jerusalem, and also in the tradition of their fathers, which were correct, who believed in the commandments of God and kept them, were called the Nephites, or the people of Nephi, from that time forth—

Mormon distinguishes between groups mentioned in the Book of Mormon so that the reader is clear as to who is who. Anyone who is loyal to gospel principles and has a testimony of the scriptures that were brought out of Jerusalem and are living the commandments are called Nephites. Those who are loyal to the false traditions and incorrect principles of the Lamanites will be called Lamanites.
I wonder if Mormon is making this distinction so the reader is not thinking that all individuals or who are loyal to the Lamanite traditions are in fact descendants of Laman and Lemuel. I think that by this time there are Nephites who have “left the fold” and are living among the Lamanites but are not distinguished by the darken skin necessarily. In fact, I think that the people that were in the land when Lehi arrived in the promised land may be joining with both Nephites, as the Nephites are probably proselyting among them or with the Lamanites because they like that life style better. In any case, Mormon will be using these terms from now on to distinguish between believers and non-believers even though the believers have many problems with believing as we will see. It should be noted however that the terms Nephites and Lamanites were used in the previous books in the Book of Mormon. It maybe that Mormon saw this as he discovered these plates as he was going through them all to see what else he might put in the record.

Alma 3:12 And it is they who have kept the records which are true of their people, and also of the people of the Lamanites.

And it is the Nephites who are keepings the histories of both the Nephites and Lamanites.
The record keepers wanted to have a record of all the peoples not just the Nephites. Someone knew that the Lamanite descendants would be reading the Book of Mormon and did want them to know of their history as well. Reading the title page of the Book of Mormon tells us that.

Alma 3:13 Now we will return again to the Amlicites, for they also had a mark set upon them; yea, they set the mark upon themselves, yea, even a mark of red upon their foreheads.

Getting back to the Amlicites, they did mark themselves just like the Lamanites who had placed a red mark on their foreheads.

Alma 3:14 Thus the word of God is fulfilled, for these are the words which he said to Nephi: Behold, the Lamanites have I cursed, and I will set a mark on them that they and their seed may be separated from thee and thy seed, from this time henceforth and forever, except they repent of their wickedness and turn to me that I may have mercy upon them.

With the placing of the red mark on their foreheads, the Lamanites were fulfilling the words the Lord spoke to Nephi. The Lamanites will be cursed and will have a mark set on them which will distinguish the Lamanites from the Nephites.
What is interesting, is that they did it to themselves. I don’t think the Lord came down and put them red mark on each of their foreheads himself. He may have inspired them to do so or allowed satan to inspire them to do so. I think we are seeing some of that today, with the large numbers of people getting tattoos and various body piercings. We are seeing a separation because I don’t see those who do this associate with those who don’t.

Alma 3:15 And again: I will set a mark upon him that mingleth his seed with thy brethren, that they may be cursed also.

Now because the Amlicites had now joined up with the Lamanites, the have also put this red mark on their foreheads. This is again fulfilling prophecy. The Lord said that anyone who mingles with the Lamanites will have the same mark put upon them. And again they did it to themselves.