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The international standard for identifying postal items
The Universal Postal Union's S10 standard, in all its glory
These are JA–JZ, KA–KZ, SA–SZ, TA–TZ and WA–WZ since these carry the risk of causing a tracking number to be confused with codes from a different UN data encoding standard. It's exactly 8 digits long and is generated by the originating carrier as they see fit, provided they don't resue the same number within 12 months. Even then, you have 10 million parcels per service indicator per country available to be issued, so it's probably not going to be necessary for a carrier to reuse a number for a much longer period of time anyway.
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