Registered in 2009 for the XPanel daemon; the UDP side is reserved and the TCP side is almost never seen.
IANA assigned TCP 3737 to xpanel, the "XPanel Daemon", registered by Lilian Rudenco in March 2009, and marked UDP 3737 Reserved. No public protocol specification accompanies the registration and no widely used implementation is associated with it. That is the whole honest picture: a name in the registry, and a port that scanners rarely find open.
Expose with care
With no documented protocol there is nothing to assess — identify the listening process and judge that instead.
$ nmap -Pn -sV -p 3737 TARGETSeen 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.