The Z-Clip planes parameters allow you to "clip" or hide from the render view certain parts of your object while still having them influence the lighting in the scene. When this parameter is active, two planes are attached to the camera, a blue plane representing the near plane of the camera - anything geometry before this plane will be made invisible in the render, and a red plane representing the far plane of the camera - anything beyond this plane will be made invisible in the render.
Z-Clip for the camera activated. In Maxwell Studio you can see the effect of the Z-Clip feature also when looking through the camera in a viewport (right image). This allows you to quickly fine tune the placement of the near/far planes.
Render of the above camera viewport
This feature can be very useful for architectural renders where you need to look inside a building but you need to show what the lighting would look like as if a wall or roof was there. Please note that the areas where no light enters the object will be rendered black.
The distance of the Z-clip near/far planes to the camera is specificed in meters. This feature is available in Maxwell Studio and most of the plug-ins.