The ATC is a two-byte counter the card increments on every transaction attempt. It is just a number — but a load-bearing one: it goes into every application cryptogram and it is how issuers spot replays and clones. Paste the hex, get the decimal and the reading.
The card increments the ATC at every GENERATE AC — every transaction attempt, approved or not, online or offline. It is a plain 16-bit unsigned integer: 003C is transaction number 60 in this application's life.
Its real job is cryptographic. The ATC is an input to the application cryptogram (and, in most schemes, to session key derivation), which is what makes every ARQC different even for two identical purchases at the same terminal. Capture a cryptogram and send it again and the issuer sees the same ATC twice — that is the replay detection, and it is the whole point of having the counter.
The card also keeps the ATC of the last transaction that was authorised online, exposed as tag 9F13. The difference between the two is the number of consecutive offline transactions since the issuer last saw this card. Terminal risk management compares that delta against the card's lower and upper consecutive offline limits — the checks that surface as TVR byte 4 bits and force the next transaction online.
Paste both values above and the tool computes the delta. A delta of 0 means the previous transaction went online; a large delta means a long offline run, which is normal for transit-style profiles and worth a look elsewhere.
Duplicates: the same ATC on two records for one card means the same transaction was submitted twice (clearing duplicates do happen) or someone is replaying a captured cryptogram. Backwards jumps: an ATC lower than an earlier one is either out-of-order arrival — offline transactions clear late, so this is common and benign — or two physical cards sharing one PAN, which is the cloned-card signature issuers actually hunt for. Zero: counters start at 1 at personalisation; a zero is almost always a read error. Near 65535: the card is running out of counter; what happens at exhaustion is a personalisation choice, and “stops working” is more common than “wraps”.