The construction of panelboards is part 2 code and the part 2 code fills a library. It is cumulatively all the standards for products. Doug has pointed out C22.2 #29 for panelboards Appendix A lists much of the part 2 publications for electrical products. It is just a listing of publications and 15 pages long or around 750 product standards. This is why inspectors don't enforce part 2. They would need a van to carry their code books. It is much easier to look for an approval sticker.
Panelboards used for a service entry must have the incoming lines separated from the branch section. It serves to barrier those unprotected Hydro conductors from prying fingers. It also give you a chance to shut the buss off when working in the panel.
In BC there is a directive that requires main breakers (combo panels) when located in a branch circuit position to have a barrier over the line terminals as an added safety feature.
When a panel is used as a service entry only service conductors Line, neutral, and ground may be in the main breaker section and the panel must be approved for use as service equipment. When a combination panel is used as a sub panel then branch wiring may pass through the main breaker section as long as the barrier cannot damage the branch wires as they go past.
