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RCOIs are not registered in a central registration authority per se, like RADIUS attributes might be (with the IANA), or a mobile network MCC/MNC pair (with the 3GPPITU.T).
However, RCOIs as they currently exist are derived from a registration at the IEEE. The IEEE is in charge of assigning MAC addresses, amongst other things, and here, some organisations have registered themselves and used some of their registration 'name space' to create RCOIs.
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