Fn (Function) Button
Explanation: The Fn button (the upper one of the two black buttons next to the lens) is a programmable function key that can get one out of many possible functions in the custom setting menu f3 Assign Fn button.
It is designed for settings that you want to have available immediately, without moving your hands and without having to take the camera away from the eye.
In Nikon‘s default settings it will activate the FV lock, i.e. send a flash for metering exposure and lock the flash exposure.
Tips: The predefined FV lock for flash exposure is a good choice by Nikon, see the recommendation for the custom setting menu
f3 Assing Fn button.