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Basicly theres this QR code feature that lets you create a Mii into a code so you can share it with other people!
When you save it, it will be located in:
SD/MMC (D:)/DCIM/1(Highest number(s)
Or atleast on Win7....
Example of a code:

Go to the mii creator then "Save/find QR code" then go to Find and hover the picture over the code image and you can get that Mii!