The RC5 socket is to enable a hard wired connection for an external IR (infra-red) control signal from an IR distribution system such as a Xantech which allows you to send IR commands to your Naim system even if it is in another room or located behind closed doors. The RC5 socket is also used by the System Automation function of the ND series products to allow control of Naim Preamps and the Naim DAC from the n-Stream control application.