User blog comment:Takazuki/Questions for Kunori/@comment-1173941-20140202231210/@comment-1173941-20140204192839

Pretty sure he means it's the ID from where they were born. So Kirito was the 6355th person to be born in the area surrounding Rulid village, which is labled "NND7". (Eugeo's ID was shown in events after Volume 13, while they were in the Tower, so yeah.)

So my guess is that Underworld is divided into sectors. "NND" = Northern N Division? Not that important, but yeah.