The orientation of the mesh defines the space to merge.
Here the inverted cube volumn will be subtracted.
So, adding an empty cube.
Due to heavy processing needs of the boolean operation, the program use simplifications.
If the result still have to be optimized for futher uses, linear functions are usefull : here the cut planes contains 1681 tris.
So we can apply optimization transforms to get a better mesh.
Optimizing.
Moreover, the volumn orientation must be considered in order to process the operation of boolean, because of complex cases of the operation.