ZmaxZmax shows the S11, (1) and (2) are first and second modes respectively. So in principle, Zmax(1)Zmax(1) shows the reflection of 1st mode when 1st mode would be incident upon the antenna.
S11, return loss which actually represents the reflection of wave that means if wave transmit from port 1 and is being reflected from port 2 that means S11 will be calculated at port 1 to get how much reflection we are having so S11 means Zmax(1)Zmax(1) .