The 84.5 mm Professional Medium Reversed Neutral Density Filter (Square, P Type) is designed to reduce exposure of the upper (darkened) part of the filter, used to reduce highlights/overexposure of the upper part of image/sky. The human eye has a much greater dynamic range than photographic film or CCD/CMOS digital sensor/digital back (dynamic range means the ability to visualize a wide range from darkest to lightest shades of the light).
Because this fact, it often happens that high dynamic scene that is seen by simple human eye after shooting with camera lose most of its dynamic range, for example dramatic textured clouds are on the digital photo visible only as a large white area, in photographic terminology: "over-exposed area." Upper part of the image is very light and lower part is very dark.
This negative fact is present because digital sensor or film cannot record images with a large dynamic range such as a human eye. Due to this, a photograph taken outside (outdoor) is sometimes useless when a filter is not used. And that is why almost all outdoor photographers (landscape, architecture or nature photographers) use filters so-called gradual ND filters (neutral density filters).
The neutral density filter is a neutral gray color, which reduces (amount of light passing through a filter) and neutrally darkens the upper part of image as far as possible without affecting its color tone (neutrality). The result is often stunning - "blow" the sky will disappear, the sky and cloud structure becomes visible, three-dimensional lattice, while the very bottom (landscape) remain ideal exposed (not over-exposed, not dark). Moreover, the effect is very natural and the use of the filter is not noticeable. Professional Lines are square shaped filters fully compatible with holders of "P" type square filters (width 84-85 mm).