This is a Graduated Neutral Density Filter used for Cinematography. 50% of the filter is clear and 50% has a Neutral Density of 0.3 (1 stop).
The transition from clear to ND is done with a HARD EDGE (HE) transition line.
Hard Edge grads lines are used with long focal length lenses where the hard transition line is softened by the long focal length lens.
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.
