What are the difference between c2,c3,c4 and t2,t3?

I have been studying Covariance matrix in SAR polarimetry and I have done a python program that computes the Covariance matrix. Now when I wanted to check whether my program is correct or not , I went to snap and found that for Covariance there are 3 options. Similarly for Coherent Matrix there are 2 options. Now I am confused which one to select. And in the python program, I am selecting each pixel and extracting pixel information. Then getting Shh,Shv,Svh,Svv, then computing the matrices using formulas. So how to validate the result?

If you open the processor dialogue and select Help / Help you can find the explanation of the options.
Only C2 is not explained.

Maybe @cwong or @timmoorhouse can add the explanation for this option.