fr4 epoxy substrate height

hi everybody
i finished my design for wimax antenna for 2.5 and 5 ghz...
in fabrication i faced problem about the substrate height
the substrate fr4 with height of -1 mm
what is the meaning of - sign here
where if the height changed to +1 mm the result changes for s11
here attached figure

The z-size is -1mm. It means that the object goes from 0mm to -1mm in the z direction. Thus it has a height of 1mm.
It is also possible to define the object with a position "x,x,-1" and a z-size of 1mm.

If you change the z-size to +1mm the object also has a height of 1mm but now goes into upper z direction (from 0mm to 1mm). This would lead to different simulation result because you change the geometry.