I just go by what panel labels say. They say different things. Here's one that gives you options and would be suitable if used in the situation in this thread (notwithstanding the other issues). Note the business about not being used and a lighting and appliance branch circuit panelboard. So you are good to feed other panels, and, I would argue, install solar under 705.12(A). Note also that there's a provision for bonding the neutral.