I am not sure how it is determined but the list is usually posted on the aircraft table.
Here it is for the series HOG
http://www.squadselectseries.com/events/HOG/HOGAC.htmlI don't know why JABO didn't include it for this series or how it's determined. It may have just been an oversight that he didn't include it.
I think it's pretty sad that JABO has to make it mandatory and assign squads to CO duties in a series.
I am not sure how it's assigned and to be honest I don't go and look to see if
JG-51 is required to CO a series. I just sign up for frame IV before someone else gets it.