The 'N' indicates the presence of a button which when pressed puts the car's transmission in Neutral. This is to assist the flag marshals in recovering cars which have expired somewhere out on the circuit and have to be pushed off or away from the track - or to a point where a crane can grab them.
Similarly, the 'E' is the emergency cut off button for all electrical power in the car, so the presence of that electricity doesn't do anything potentially harmful to anyone involved in a vehicle recovery situation, or more importantly so the car doesn't electrocute anyone who might be trying to rescue the driver from the wreck, or by sparking start a fuel fire.
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