The exposed (Editable) variables give designers control over the following properties of the Material:
The color of the fog effect.
Brightness applied to the fog color.
How well the fog can be seen through.
Depth Fade Distance
Controls the distance away from intersecting geometry that the effect will fade. This prevents harsh cuts along walls and other surfaces.
Noise Projection Distance
Provides a depth offset for fog noise.
This is similar to tiling - the higher the value, the fewer tiles of the noise texture.
How quickly the noise texture pans across the fog surface.
The distance between the player and the effect at which the fog fades as the player approaches. This prevents the player from noticing the sheet geometry.
This controls the size of the sheet mesh used to display the fog sheet.
The Size variable is particularly interesting in that it has had its 3D Widget enabled. The 3D Widget is a feature available only to vector variables, and creates a visible object in the Viewport that can be manipulated to change the 3D vector value.
Since a 3D Widget is simply a mechanism for controlling a Blueprint variable, the widget is only visible when it is associated Blueprint is selected. The widget can then be selected and moved using the standard Move tool. This provides a designer with a quick way of changing a vector value.
In this case, the 3D Widget is employed to allow for quick resizing of the fog sheet geometry plane. The upper right corner of the mesh meets the location of the 3D Widget, and the sheet as a whole scales from the center, where the Blueprint's sprite icon is located.