Nintendo 3DS

I plan on getting a 3DS eventually and my friend has one but I was just wondering how the friend codes work. I ask as I read that the code is assigned to a 3DS system but my friend has two Mii accounts on their 3DS (one for himself and the other for his wife) since they both share it.

Are the friend codes tied to one system or are they tied to a Mii account? Sorry if I'm sounding confused as like I said earlier I'm only basing this all on hearsay since I myself don't have a 3DS yet. I'm just curious as to how it would work when I eventually get one.