Password - Yugipedia - Yu-Gi-Oh! wiki

A password is the eight-digit number printed in the bottom-left corner on most OCG/TCG cards (certain cards, such as "Gate Guardian", do not have a password, and some cards instead have limitation text in this spot). The password for a card is always the same in all languages and all prints of that card, including alternate artwork versions, except for two ("Dark Magician" has an Arkana version, and "Polymerization" has an alternate artwork version, both with different passwords from their main versions). Because of this consistency, passwords can be useful for identifying cards.

Yugi Muto, Yu-Gi-Oh! Wiki

Secret Password - Yugipedia - Yu-Gi-Oh! wiki

Evil☆Twin, Yu-Gi-Oh! Wiki

Battleguard Cadet - Yugipedia - Yu-Gi-Oh! wiki

Passcode, Yu-Gi-Oh! Wiki

Simul Archfiends - Yugipedia - Yu-Gi-Oh! wiki

Masked Clown - Yugipedia - Yu-Gi-Oh! wiki

Yu-Gi-Oh! 7 Trials to Glory: World Championship Tournament 2005, Yu-Gi-Oh! Wiki

Yu-Gi-Oh! 5D's World Championship 2011: Over the Nexus, Yu-Gi-Oh! Wiki