This can not be the typical split bus panel we have seen in past where there is one set of main lugs that is supplying two separate sections of bus. It is describing a panel with multiple feeders supplying it. I can not recall ever seeing such an animal.
The panel I was initially talking about was the ones we used to see that had main lugs that fed 12 poles which if you installed six 2 pole breakers you have your allowed service disconnecting means. One of those 2 pole breakers served as the lighting main for the second bus section in the panel. The panel only has one supply but is split possibly six ways.
The panel that Augie submitted a schematic for I have also seen - but they were all old Pushmatic 200 amp panels. They had 200 amp main lugs that supplied two 100 amp main breakers - each supplying a separate set of bus bars. It was usually a left set and right set instead of top set and bottom set. This panel still only has one feed to it and is then split. Maybe I don't read it right but the way it is written I understand the exception in question to mean the panel has two supplies to it. Take the schematic for the one Augie submitted. It likely has 200 amp main lugs and 200 amp bus on supply side of the main breakers. Is that not still an extension of the feeder supplying the panel? Then after each main we have two sub feeders of 100 amps each protected by 100 amp OCPD. Is this one panelboard or two within same cabinet?
But then we have UL guys and CMP info that suggests this is what it was intended to apply to so :slaphead:
I don't think you will find such an animal with any new equipment, and the older ones that are still out there were likely limited to 42 circuits at the time they were made so I don't think it is really much of a problem anyway.