WHAT DOES KEY RETURN ZIP CODE MEAN

[coldcast55]

I SENT THE CUSTOMER PACKAGE OUT AND IT WAS RETURNED TO ME WITH THIS STICKER

 RETURN TO SENDER    KEY RETURN ZIP CODE 

[simba6]

In our experience the white “KEY RETURN ZIP CODE” label is used to prove the package was successfully sent to the US through customs but was being returned to the sender in another country. This sticker is to prove the package was sent to the zip code on the address but was not delivered.

[martin2001]

I don't have any experience with this sticker, never heard of him. Not much info (practically none) on the web. There are, for some reason, many  PDF documents showing images of various envelopes that were not delivered and could not be forwarded and were to be returned to sender. I only saw the images for the domestic mail. I am attaching one of them. It looks like the barcode on this sticker contains the sender's ZIP code.  One can only speculate why there is the word KEY on the sticker, why not only RETURN ZIP CODE.

[deutches-philatelics]

Wild guess?    Internal use for USPS.