Yea, PO Boxes generally have their own Zip Code so they use +4 for that.

However, if you look at zip+4 for dwellings, it’s still few. My cul de sac with 5 houses has zip + 4 different from house on connecting street.

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dhosek 2 days ago | parent [-]

It’s based at least in part on delivery routes.

There’s a less-known 11-digit zip code which is unique for every delivery point (so down to the individual residence). I’m not sure if multiple apartments in the same building have distinct 11-digit zip codes, but this does imply that a zip+4 cannot have more than 100 delivery points within its bounds.

	▲	stackskipton a day ago \| parent [-]
		Apartments will commonly have multiple Zip + 4 for the building so I imagine each apartment gets unique Zip 11. Open Google Maps, go to Central Park in NYC, search for apartments and randomly pick one. Then go USPS Zip Code lookup (https://tools.usps.com/zip-code-lookup.htm) and punch in the address leaving off any apartment number so it will show all available addresses. I used 225 E 63rd St New York, NY. Appears they have 8 Zip + 4 assigned to the complex. I tried again with building in Philly and same story. Each floor of 16 apartments got its own Zip + 4.