42

Issuer Identification Number is the classic BIN/IIN prefix embedded at the start of the PAN (5A, referenced alongside it) that names the card's issuer and major industry - the same prefix payment networks have used for routing and issuer identification since long before EMV existed, now also available as its own standalone tag rather than requiring every system to extract it by parsing the first several digits out of the PAN directly. It's related to the Bank Identifier Code (5F54), another issuer-identifying field defined for a different purpose (international bank-to-bank routing rather than card-network routing), and to the Issuer Identification Number Extended (9F0C, covered earlier in this dictionary) that exists specifically because this classic field's digit count is running out of room for how many issuers the industry now needs to distinguish. A terminal or host that reads 42 instead of parsing it out of 5A gets a cleaner, more explicit answer to who issued this card without needing any assumption about exactly how many digits of the PAN the IIN occupies, which can genuinely vary. See EMV 4.4 Book 3.