questions about s21,spread delay

hello,

what is the difference between spread delay and group delay ? is it not the same ?

and s21 is the relation between complex output wave and input wave is that correct ? so its something i could explain like this : "how much of my transmitted wave comes in at my receiver" or how much of the initial power is left ?

another question is, how can i interpret the phase of s21 ? i know its complex so it has a phase but what can i gather from a s21 phase plot ?

thanks in advance

I don't know about spread delay, but essentially, yes, S21 measures the received signal with respect to the sent signal. This is a complex number; the phase is just how much phase a wave has incurred as it travels (so it's related to distance, in an empty medium).