"Fill Using Value" sets all pixels (DEM values) under the mask to the value specified by the user in the "Value" text field.
For example, using the Mask Operations the user traces and fills a mask over a lake. The elevation of the lake is read off a map and entered into the "Value" text field. The "Fill Using Value" button is then used to set all the DEM values for the lake to the known lake elevation.
"Fill Using Average" is similar to the "Fill Using Value" operation except that all the pixels (DEM values) under the mask are set to the average (mean) value of the elevations under the mask. The current average value is shown to the right of this button.
For example, the user knows that a particular residential area is basically flat; however, the DEM has many bumps due to buildings etc... The user masks the residential area, then uses the average elevation for the area as the new elevation. This removes the bumps.
"Fill Each Polygon with Polygon Average" is similar to the "Fill Using Average" function; however, each individual polygon in the mask is identified and its average is calculated independently and used to fill the polygon. If there is only one polygon in the mask, this is equivalent to the "Fill Using Average" function.