6F

Tag 6F is the outermost wrapper the card returns in response to SELECT — the very first constructed template a terminal parses in a transaction. Inside it lives the File Control Information: the application identifiers, labels and discretionary data that let the terminal know which application it has reached and how to talk to it. Get this parse right and the rest of the transaction has a foundation; get it wrong and nothing downstream can be trusted.

Reading it is exactly what the decoder on this page does: open the value as nested BER-TLV and list the constructed and primitive objects inside. A typical FCI carries the Dedicated File Name (84), the Application Label (50), the Application Priority Indicator (87) when multiple applications coexist, the FCI Issuer Discretionary Data (BF0C) for anything the issuer wants to add, and the FCI Proprietary Template (A5) which itself contains application-specific objects like the Processing Options Data Object List (PDOL, 9F38).

Why this matters is that 6F is the entry point to application selection, and application selection is the entry point to everything else. PPSE (Proximity Payment System Environment) SELECT returns a list of 6F templates, one per candidate application; the terminal chooses based on priority and capability, then issues a final SELECT whose 6F response gives it the PDOL it must satisfy in GET PROCESSING OPTIONS. A parser bug inside 6F — mis-reading a nested template, dropping the BF0C — corrupts the selection itself, and the symptom is usually a terminal that cannot find an application on a card that has several.

The integration trap is treating 6F as a flat list of tags. It is constructed and nests: A5 (FCI Proprietary Template) is itself constructed and carries its own children; BF0C is constructed and carries issuer discretionary data that may itself be nested. A parser that reads only the top-level children of 6F and does not recurse into A5 will miss the PDOL entirely — and without the PDOL, the terminal has no way to know what to send in GET PROCESSING OPTIONS, and the transaction stalls at the next step.

For decoding, treat 6F as the root of a tree and recurse. Every constructed child (A5, BF0C) must itself be opened; every primitive child (84, 50, 9F38) is a leaf with a value. A BER-TLV parser that handles multi-byte tags, long-form lengths and arbitrary nesting depth is not optional here — the FCI is the first place a naive parser that assumes single-byte tags and flat structure will fail, and the failure is silent (missing data) rather than loud (parse error).