Could someone explain the Layout layers of ADS?

In ADS examples: \hpeesof\examples\Momentum\Microwave\Waveguide_prj ,
the substate define is as follwoing: all the three middle layers under "layout Layers" defined as "cond"..."Perfect Conductor".
I want to know why the "air" layer has such definition?
Attached pictuer:

I finally found that the software is sometimes lag, for air layer, its value is displayed by the last other layer's display.
It doesn't affect the last result because there is no definition on that layer.