The top and bottom machining levels define the machining range along the Z axis. Only those areas of detail surface will be machined, which lie between the top and bottom levels.
If the workpiece or the restricting model are assigned by areas, which lie in the basis of a prism, then it is considered that this rule extend to all machining levels, i.e. between the bottom and top machining levels.
Off course, the top machining level cannot be lower than the bottom one.
The top and bottom machining levels can be assigned in the <Parameters> page.