How to interpret the S-parameters of this result?
According to the CST MWS help page on FSS, it is mentioned that the reflection is specified using SZmax(1),Zmax(1) while the transmission is specified using SZmin(1),Zmax(1). When the magnitude of the reflection is greater than the transmission by a fair margin, then the designed FSS is said to function as a ground plane for the designed band.