In the US, apartments have numbers or maybe letters. Sometimes they have fractions. So you could get an address like 123 South Street, Apt. C, or 321 North Street, #33 1/2, or good ol' 678 West Avenue, #202.
It turns out that such a simple idea for identifying individual units without needing names is a good idea - and not implemented everywhere. Like in Norway.
I have been in a few high rises that use apartment numbers, but that's so you know which button to push to get buzzed in. In low buildings like mine (4 stories, 3-4 entries a building, 6-12 units per entry), there are no apartment numbers. You cannot be anonymous. You have to put your name on the door.
You still need a name on the door, but now Norway has found that it is useful to be able to identify apartments independently of whichever name(s) are attached to it (an issue common in a country where so many co-habitat without marrying or taking the husband's last name). And now they've made it mandatory; it has to be on the deed, too.
The Norwegian IRS sent me a letter about the requirement of apartment ID, with a sticker that goes on the doorjamb somewhere, and optionally on the mailbox. If I didn't know my number (I didn't because the original paperwork from 10 years ago is whoknowswhere), I had to call the government map agency. That, after a lengthy wait, turned out to be one of the nicest phone conversations I've had in a long while. The man who took my call had the right mix of humor, friendliness and efficiency.
So I am now registered with H (residential floor) 01 (first floor) 02 (second door on landing). Stickers are on their way in the mail.