One credit is one question, asked of one engine, for one location, fetched fresh. If you track 20 questions across 5 engines, a completely fresh sweep is 100 credits.
Most scans cost far less than that, because answers to general questions are shared. If somebody else recently asked ChatGPT “best mobile car detailing in Los Angeles”, you get that answer free. The estimate screen shows you exactly how many cells are cached and how many are chargeable before anything is spent.
What never costs a credit
Technical checks. Re-run them as often as you like.
Cached answers, however many.
A cell that failed because an engine errored. It is refunded automatically.
Regenerating your Fix Pack, or exporting a report.
If you run out
Nothing is taken away. Your reports, history and Fix Pack all still work, and technical re-scans still run. What you cannot do is fetch fresh answers until your credits reset on your licence anniversary, or you buy a top-up, which is optional.
Your billing page shows every credit movement individually, with what caused it. If a number there looks wrong, that list is the place to point at.