Unassigned at IANA with no established occupant — identify it locally, not from a port list.
There is no IANA service registration for 9643, and Nmap's frequency data puts it among the rarely-observed ports. No widely deployed product uses it as a default and there is no notable published malware association. Anything found here is application-specific.
Expose with care
With no known protocol there is nothing to vouch for. Determine what answers before allowing it through a firewall.
$ sudo lsof -nP -iTCP:9643 -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.