It’s such a little thing, I mean physically, it’s darn small. And surprisingly many Z 9 owners are not aware that it’s even there. The red arrow is pointing at the eye-sensor on the Z 9. Its main purpose is to sense when the eye is at the viewfinder. Depending on the EVF mode you’re in, it will switch the view between the EVF and the monitor. I’ve now come across a number of Z 9 owners who assumed their Z 9 needed to go in for repair when it was just the eye-sensor was being blocked. All but one was because the sensor had become dirty. The exception was an eyecup was blocking the sensor. So, if you expect the LCD to display an image when you move your eye away from the EVF and it doesn’t appear, look for blockage cause the eye-sensor, it needs to see!