This is a value that represents how much of the viewport is occupied by the character's bounding box (By default, the bounding box is derived from the Physics Asset, if one is present, and Skeleton Asset if not.). Factors that affect this number are how large the animation's bounding box is (or the displacement in 3D space), how close the character is to the camera, and the amount of zoom applied to the camera. Based on this value you can assign when your LOD should switch between levels.