It runs the same calls the app makes — sign-in, both libraries, the register, the customers, the toolmakers, the payment terms, then a real folder, a real upload and a real download link — and says which step fails and with what error. Read-only except for one test file it writes into _selftest/, which it names at the end. Nothing is deleted and no mail is sent unless you press the mail button.
Anybody with the app open in the last 4 minutes counts as online. A closed laptop stops writing and drops off the list by itself — that is why somebody can show as „last seen 20 min ago" rather than offline.