insertion loss and attenuation

With respect to limiter and linearizer,when it is appropiate to use insertion loss and when we should use attenuation?

In limiter the high power signal decreases.So is it termed as" attenuation has increased or insertion loss is increased?

Similarly in linearizer circuit,the gain increases at high power levels

So what to use here? insertion loss has reduced or gain has increased?

what is the difference between insertion loss and attenuation?
what is the difference between gain and S21?

what is the difference between insertion loss and s12?

please help me.

Thank you in advance

|S21| dB is the insertion loss but I would call it gain (power gain) for an active circuit/amplifier >0dB and will call it insertion loss if passive/lossy circuit <0dB

For passive circuits/reciprocal S21 = S12.. This is not the case for active ones. For instance, amplifier magnify the signal in one direction but isolate the leakage from output to input

Please read a basics textbook on S parameters definitions. I can try to give you answers but if you know nothing of the basics you can be confused.

S parameters can be given other names like insertion loss or attenuation for S21, S12 backwards, gain can be expressed as negative loss, etc.
Best do start studying the basics, please.