Your DS keeps a list of Miis that you've met while walking around. The list only has 10 spaces, & stores the first 10 Miis you meet. Instead of greeting new Miis at the Plaza Gate right away, now you can move them to the Line, and greet them later. The new Miis are brought into StreetPass MiiPlaza, and taken off of the list your DS keeps. You can then go out and your DS will record 10 more Miis.