Ask in plain English. The answer is assembled from this site's own pages and always shows which ones it used, so you can check it rather than trust it.
This box goes to a server. The parser does not.
Everything else on this site runs in your browser — the parser never uploads your message. This assistant is different: your question is sent to a server to be answered. Do not paste real card numbers, real cardholder data, or a production message. Describe the field or the code instead, or use the local parser for real data.
Limited to 20 questions a day per visitor — enough for real use, and enough to keep the endpoint from being farmed.
What it can and cannot do
It only uses this site. If a question is not covered here, it says so instead of filling the gap from general knowledge. That is deliberate: field widths, code values and edition attributions differ between the 1987, 1993 and 2003 editions and between card schemes, and a confident wrong answer is worse than no answer.
Check the sources. Every answer lists the pages it drew on. For anything you are going to act on, open them — the page is the authority, not the summary.
It cannot parse your message. It has no access to your data and no parser. To decode an actual message use the parser on the home page, which runs locally.
It does not know your scheme specification. Private fields such as DE48 and DE60–DE63 are filled differently by every network, and those documents are not public. It will tell you that rather than guess.