9F7C

Merchant Custom Data is a fixed 20-byte field the card can request from the terminal during a Kernel 2 transaction, carrying whatever merchant-specific content the issuer or scheme program actually needs - it's related to the Data Storage Requested Operator ID (9F5C, covered alongside it here) as part of the same broader Kernel 2 data-storage feature that lets a merchant or program operator exchange lightweight custom data with the card at the point of sale. Unlike most terminal-supplied tags in this dictionary, which carry values with a fixed, EMV-defined meaning, 9F7C's content is entirely program-specific - a loyalty identifier, a merchant program code, or any other 20 bytes a particular scheme's data-storage program defines - so a terminal populates it only when the card's own request, via the relevant DOL, actually calls for it, not as a matter of course on every transaction. A terminal or host that doesn't participate in the specific data-storage program a card expects will simply never populate 9F7C, and a card that doesn't get expected values here typically falls back gracefully rather than failing the transaction outright. See EMV Contactless Book C-2.