FULL NEUTRAL DENSITY FILTERS have the same density throughout the filter which means that the light is reduced evenly throughout the entire image. ZERO Filters currently produce Full filters with strength ranging from ND4 to ND32 which is 2 to 5-stop reduction of light.
Using square full neutral density filters requires using filter holder. Some photographers who do not combine full and graduated or polarising filters prefer using screw on circular full ND filters because they find using these filters more convenient. However, just like almost everything else, this depends on the workflow of the photographer. Both filters have certain advantages and disadvantages. The main advantage of using square ND filters is the fact that they are easy to combine with graduated neutral density filters or polarising filters without introducing vignetting when shooting wide. Personally, I find using square filters somewhat easier, especially when using very dark filters which make framing a shot very difficult because they are hard to see through when they are in front of the lens.
In landscape photography, full filters are most commonly used to achieve long exposure times when photographing flowing water or moving clouds. Correct shutter speed can help to create dynamic images with sense of movement or it can create sense of stillness and give images very ethereal feel. However it can also create tension between motion and stillness. Very long exposures can sometimes help simplify a scene by eliminating detail in moving water or clouds which helps to bring attention to other elements in the image.
Same techniques can also be applied to cityscape photography. Very long exposures can help create light trails giving an image dynamic character or they can be used as leading lines bringing the attention of the viewer to the main subject of the image. However, they can also be used for photographing people to emphasize movement or creating surreal scenes of ghost towns by blurring people more significantly.
Surely enough, wherever there is movement, whether it is architectural photography, street photography, portraits, light painting or intimate natural scenes, there is an opportunity for creative use of full filters. And if there is no movement, you can use full ND filters to introduce movement into your images and give them dream like character by intentionally blurring out your shots by moving your camera during exposures.