Significance of S21 plot in Smith chart

Hi,

What is the significance of plotting S21 in Smith chart? I know that it plots normalized (S21-Z0)/(S21+Z0), what is its significance?

Plotting S11 makes sense because in that case it represents reflection co-efficient and hence it is helpful in matching etc. But why S21?

You are right: S21 is usually not shown in Smith chart format, because that presentation doesn't make much sense for transmission.