What is the bar on a CBP automated passport control (APC) slip?

Warning: This answer is based off of strong speculation and contains no real sources.

Border crossing technology is either really dumb, or at the forefront of data mining and AI. It either processes your declarations form and verifies your documents, or it's using facial recognition, behavioral pattern matching, and data mining to assess. Basically, it's working to replace the officer with both the mundane paperwork and validation, and the "gut feeling" officers use to determine if the person warrants further interrogation.

So, while the cynical might say it's a threat risk assessment, it is probably a confidence assessment, which is a bit more benign. Instead of it being a measure of your risk to the U.S. it is probably a measure of the computer's ability to determine if you are legitimate. So a high number is not so much of a "he's a threat," as much as it is "I don't have enough information and I need a human's help with this person."

Logical guess is the bar and FM = Face Match. It's how well the picture matches the photograph stored on the RFID chip of the passport.

It has been reported that the APC kiosks match this:

This requires that APC kiosks authenticate identity by matching people’s faces to the biometric record in their e-passport...

“Now, there is added security as the CBP requires facial biometrics to be matched to the e-passport being presented. Almost 500 million e-passports have been issued globally; these hold facial biometrics while some also contain fingerprints. Our SITA APC kiosks comply with all the latest requirements and help bolster more efficient security checks at the airports.”

I had the same question, and have a new possible answer to posit: I think it's in reference to your height. Mine was 29, my husband's was 48, and we're about 18 cm apart in height. Since the kiosk automatically adjusts its height to take your picture, it's a possible explanation. Also, my height in cm is about 158, and his is about 178, so possibly the measurement begins at 130cm at the bottom of the bar.

If my assumptions are correct, then that would make the gentleman in the picture 200cm, or 6.5 feet. Perhaps he could confirm his height for us? And if anyone else has an example, we could easily validate or invalidate this answer.