depthorder

The option set object N depthorder applies to 3D polygon objects only. Rather than assigning the object to layer front/back/behind it is included in the list of pm3d quadrangles sorted and rendered in order of depth by set pm3d depthorder. As with pm3d surfaces, two-sided coloring can be generated by specifying the object fillcolor as a linestyle. In this case the ordering of the first three vertices in the polygon determines the "side".

If you set this property for an object that is not a 3D polygon it probably will not be drawn at all.