This is the formula for the volume (light grey area) of a right circular cone with a frustum defined by base radius (a) and top radius (b) and height (h) in between where the cylindrical core (dark grey area) has been removed. This formula is simply the volume of the frustum of a cone minus the volume of a cylinder that would inscribe it. This shape, upside down, is often used measure the volume of a bevel taken off the top of a cylinder.
The chamfer put on top of a cylinder of an engine is a common use of this shape.