Code 47
In Starfleet signal protocol, a Code 47 was an encrypted communication intended for receipt only by officers holding the rank of captain or higher. Voiceprint authorization by such an officer was required to open the channel, and no computer records of the transmission were maintained.

In 2364, Captain Walker Keel of the  used a Code 47 to contact Jean-Luc Picard of the  in order to request his presence at Dytallix B. ()

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