I seem to recall somewhere a panel that was rated for a certain total rating of all of the breakers attached to it. I don't remember which, and I don't see why (if the calculated load and the feeder rating is less than the ampacity of the bars, who cares what the total of the CB ratings is), but if that is the listing, then that is the listing.
I have also seen panels where the instructions specifically limited the total rating of CBs connected to a single stab on the bus. This is more reasonable, since the rating of the stab might be less than that of the entire bus.
-Jon
