regarding background material of voxel in CST

HI everyone ,
I have one question regarding voxel use in CST.

in material file to create voxel if we do not define material 0 (background material for voxel) then background material of the CST environment is considered as the background material for voxel created. if the background in CST environment is air then background of voxel model is also air.
if this background area of the voxel model overlap with any other structure of CST then what would be the material property of the overlapping area.
i simulated like this but do not receive any structure overlapping warning but i doubt the result.

please help