question about ADS linearization design guide

I have some questions about ADS Linearization/Digital Predistortion (Analog/RF) / Step by Step design :

Step1: CSRC4 and CSRC6 both have five coefficients (Alpha_3rd,Alpha_5th,....,Beta_3rd,Beta_5th,....)
What's the theoritical base for it? Why do this design guide use 5th order polinoms for predistortion's coefficients.

Step5,6: what's role of the pilot tone? Can we analyze the effect of the disturbs near the band with it or what is it good for? If somebody knows some article where I can found the theoritical basics for this pilot tone, it will very good.

Thanks,