No registration, no documented default — whatever is here was chosen locally.
IANA has no assignment for 8540 on either transport, and Nmap tracks it as "unknown" with an open rate near 0.015%. Nothing widely deployed documents it as a default port. As with the rest of the unregistered 8xxx range, an open 8540 tells you only that some process bound it.
Expose with care
Treat it as an unidentified service and judge exposure by what the process actually is.
$ sudo lsof -nP -iTCP:8540 -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.