最新のカリングのパスの後にある、スフィアのディスタンスバンドのインデックス
Note that this value will range from 0 to the length of the distance array passed to CullingGroup.SetBoundingDistances inclusive, as each bounding distance in the array is an upper bound for that index.
For example, given bounding distances [10, 20, 30], the first band (index 0) is spheres that are between 0 and 10 units away from the reference point, the second band (index 1) is spheres between 10 and 20 units away from the reference point, the third band (index 2) is spheres between 20 and 30 units from the reference point, and the implicit fourth band (index 3) is spheres beyond 30 units from the reference point.