Usually the second app on a developer's machine, after something already took 3000.
3001 has no meaningful standard behind it: it is what a Node, Rails, or Vite process binds when 3000 is occupied, so a full-stack repo commonly runs an API on 3000 and a client on 3001 or the reverse. IANA registers TCP 3001 to OrigoDB's native interface (the UDP side is Reserved), and older scanner databases label it Nessus, from a daemon port that Nessus has not used in decades. On a scan it is nearly always a development or internal HTTP service.
Expose with care
A production app behind a reverse proxy is fine; a framework dev server on 3001 has debug endpoints and verbose errors and should never face the internet.
$ curl -sI --max-time 5 http://TARGET:3001/Seen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.