What are the white bar code stickers I see on rear passenger windows of cars?

What are the white bar code stickers I see on rear passenger windows of cars?
I see them on all makes and models of cars around town and I have no clue what they are for as they have no label or identifying information on them!
These are all makes and models of cars that I see driving around the bay area (Northern CA). It’s not like they all come from the same dealer or work in the same place or even live in the same community…

Best answer:

Every thing has a tracking number, the bar code are the same. UPC Codes.

Tags: cars, code, passenger, rear, stickers, white, windows

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