Registered to the Object Management Group for CORBA bootstrap references, and almost never actually used.
The IANA registration is omginitialrefs, from CORBA: an ORB could publish its initial object references — NameService, RootPOA and friends — at a fixed well-known port so a client could bootstrap without being handed a corbaloc URL. Real CORBA deployments configure their own ports, so 900 sits empty on essentially every host. Anything you find listening there today picked a round number for itself, usually an internal service or an admin web UI. nmap's frequency data puts 900 far down the list of ports found open, which matches that.
Expose with care
No standard service runs here, so an open 900 is an unidentified listener — fingerprint it before deciding anything, and treat an unknown service on a public IP as a finding rather than a feature.
$ sudo lsof -nP -iTCP:900 -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.