Guide number (Gn)
This is a measurement of flash power, usually quoted in metres at ISO100. Dividing the Gn by the aperture value gives the distance the light will reach and still Illuminate the subject sufficently. At ISO100 a flash with a Gn of 60 will be able to illuminate a subject 15m away at an aperture of f/4 (60÷4=15).
To enable TTL flash metering, a flash extension cord needs to be Nikon-dedicated. Examples include the Nikon SC-29.