This is used to force closure of the resulting contoured surface. Note, however, that large cap values can generate weird surface normals in those cells adjacent to the boundary of the dataset. Using smaller cap value will reduce this effect. Another important ivar is MaximumDistance. This controls how far into the volume the distance function is computed from the input geometry.
Small values give significant increases in performance.
However, there can strange sampling effects at the extreme range of the MaximumDistance. If not explicitly defined, the model bounds will be computed. This is useful when you have multiple datasets and want to create a conglomeration of all the data. Specify distance away from surface of input geometry to sample. Smaller values make large increases in performance.