The hard gradation is perfect for horizons, or any hard transition between the sky and land. These filters give great control over the exposure.
The soft gradation is ideal when there is no definite transition between sky and foreground, for instance woodland, mist & interiors.
The medium gradation is perfect to filter the sky with minimal impact on the land. Ideal when elements like rooftops and mountains protrude into the sky.
The very hard gradation is perfect for very defined horizon lines such as seascapes. The very sharp transition allows for precise separation between areas of the image.