Unassigned at IANA; in practice the second port in Swoole's multi-protocol server example, right next to 9501.
There is no IANA registration for 9502. Swoole's README shows a mixed server that serves HTTP/WebSocket on 9501 and adds a raw TCP listener on 9502, which is why the pair shows up together on hosts running PHP coroutine servers. Outside that, 9502 is an ordinary unclaimed high port that an application may have chosen for itself. Identify it by what answers, not by the number.
Expose with care
Typically a raw TCP application port with whatever authentication the app implements — which is often none. Keep it on a private interface until you know what protocol it speaks.
$ sudo lsof -nP -iTCP:9502 -sTCP:LISTENSeen 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.