It’s so they can print the tracking dots.
For the unaware: https://www.snopes.com/fact-check/household-printers-tracking-code/
Wow, I had no idea. I thought the joke was needing full ink in an unrelated color. I didn’t know about tracking. I’m sadly unsurprised.
It’s for the purpose of serialization for counterfeit purposes. Also, high end copiers have a device installed called the BDU (Bill Detection Unit) that all scans pass through before being post processed. If the BDU detects a bill being scanned it can error and shut down the whole device until the manufacturer can send someone out to fix it. I used to be one of those people resetting BDUs at schools where a teacher thought it was a good idea to copy images of money for teaching students.
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