50% of the filter is clear and 50% has a Neutral Density of 0.9 (3 stops).
The transition from clear to ND is done with a SOFT EDGE (SE) transition line. SOFT EDGE grads lines are used with wide angle and normal focal length lenses. Graduated Neutral Density filters are used to balance exposure within a scene. One very common use is to balance the exposure of sky and earth in a landscape, avoiding overexposed blank white skies.