[fn-mote]

> Zip+4 codes also change frequently because they aren’t based on locations but on delivery routes and sequencing.

This was news for me. I know the few zip+4 I memorize never change.

I think the source for the parent is AI slop. See [1].

> Due to an increase in population or to the improve postal operations, the US Postal Service® will occasionally add a new ZIP Code or change ZIP Code boundaries.

The plus four digits encode:

> [67] : Sector or Several Blocks

> [89] : Segment or One Side of a Street

Note that this contradicts the parent.

[1]: https://faq.usps.com/s/article/ZIP-Code-The-Basics

[twodave]

The census bureau (very) periodically publishes zip code data (which is where some places get their geolocation info). If you work with enough addresses you’ll find some zip+4s that are wildly far away from where they used to be. There are paid services that have better accuracy, but I’m not sure how they acquire their data.

[bombcar]

Some people don't realize just how much you can "customize" deliverability with the post office, especially if you're big (like a school or large business) - you can have something that looks like your physical address, but is actually really a maildrop/PO Box at the nearby post office.

You can do relatively complex forwarding that would only appear to the end users if they can decode the barcode.