从螺柱端面所在平面到槽后端的距离，沿平行于螺柱轴线的方向测量。如果槽是锥形的，则该距离测量至锥尖处。
The distance from the plane of the stud surface to the back of the slot, measured parallel to the stud axis. If the slot is tapered, the distance is measured to the apex of the taper.