I assume that you are referring to the tag ID (TID) sometimes written to an RFID chip during manufacture. To my knowledge—and readers are welcome to correct me if I am mistaken—there are no international standards governing how chipmakers write TIDs. A chipmaker can use 48 bits, 64 bits or some mother amount. There is also no standard regarding how those TIDs are created.
GS1, which created the Electronic Product Code (EPC) standard, does have some rules regarding TIDs, to ensure that they are not duplicated on EPC tags. You can download the EPC Tag Data Standard to learn more about this. But this standard only covers EPC Gen 2 high-frequency (HF) and ultrahigh-frequency (UHF) passive RFID tags, as well as passive HF tags complying with the ISO 18000-3 Mode 3 (3M3) standard, and does not apply to, say, passive HF tags based on the ISO 15693 air-interface protocol standard.
—Mark Roberti, Founder and Editor, RFID Journal
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