[dhosek]

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]

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.