86

Issuer Script Command is the actual instruction inside an Issuer Script Template (71 or 72, both referenced alongside it): a single post-issuance command such as PUT DATA (write a new value into a specific tag on the card) or APPLICATION BLOCK (disable the application entirely), sent from the issuer's host through the terminal to a card the issuer has no other way to reach directly. A script can bundle several 86 commands inside one template, executed by the card in sequence, and because these are genuine state-changing commands, not just data being read, a script failure partway through is meaningfully different from a read failure elsewhere in the transaction: the card may be left in a partially-updated state, which is exactly why the related IAC script-failure bits, referenced from 9F0D/9F0E/9F0F, distinguish failure before versus after the final GENERATE AC rather than treating all script failures the same way. A terminal's only real job with 86 is transport - relay the bytes faithfully and report back whether the card accepted them - the actual command semantics are between the issuer and the card. See EMV 4.4 Book 3.