[Cin] Mixers... Mix Masters

Hello friends of Cinelerra, I am going to write a tutorial on the multicamera technique, I am looking at the Cinelerra GG manual for documentation, but I cannot find information about the command "Mix masters" from the Mixers menu ... Someone would be so kind to tell me, briefly, what this option is for, or the section of the manual that talks about it. I guess it must be to indicate which track will be the master, but I don't know exactly the steps to follow to make it work correctly. Thank you very much in advance.