Nikon A Reference Manual - Page 98

Subject-tracking AF: Position the focus area over your subject and press J. The focus area will track the selected subject as it moves through the frame. To end tracking, press J again. Focus area Note that the camera may be unable to track subjects if they move quickly, leave the frame or are obscured by other objects, change visibly in size, color, or brightness, or are too small, too large, too bright, too dark, or similar in color or brightness to the background. N 78