As shown in the description for the sight, the E button should be pressed to activate the rangefinder function. However, this does not work. When cues have been enabled after pressing the U key, the text in the bottom left of the screen indicates that the button to activate the rangefinder function is R, instead of E.
#Brightsight Rangefinder Bow Sight has conflicting text description
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This is not actually a bug but something that they changed. From patch notes:
"The keybind for activating the Rangefinder bow sight was the same as the keybind for spotting. The bow sight is now “R” when aiming with a bow, or Y / Triangle on a controller."