@gorol I worked for a company doing bulk mailing in Australia, these barcodes contain the Delivery Point ID which helps in automatic sorting equipment, so they get a discount sending letters that have it. This data is just a numeric value that represents the address underneath it. But on longer ones you can add your own custom customer data which could be anything. Auspost has details here https://auspost.com.au/content/dam/auspost_corp/media/documents/barcoding-fact-sheet-oct14.pdf
@drlabman I have a really surface level knowledge about it, but what's in the barcode really depends on the country. For example in the UK as pointed out by @vfrmedia they are printed by the Royal Mail and are only used to simplify mail sorting. Something similar is used in Poland, although I'm not sure what exactly is encoded; all of the tracking of priority mail is done through a separate sticker with CODE 128 barcode.
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