This is section 9:
https://1drv.ms/b/c/18094234b935aa79/ESTwznvf6MBOgptGaHMB3fsBIOqMIT4mvZ1_IX6GyNrPRw?e=Mbvh6O
The next step should be combining FS and the paper model.
UPDATE 1;
It is said that the c plane is defined for approaching photons only (because relative v for receding photons is undefined), also it was said that only the mult inverse is defined in the c plane. So since there is no additive inverse, then the c plane is a half-plane because there is no additive inverse to map the positive portion to the negative portion. these two things are connected, meaning that receding photons are undefined because there is no additive inverse.