In airline parlance, this is called "buyer furnished equipment". Boeing sells the airframe (without the engines, even if there's only one choice) and the seats, lavatories, etc. are all provided by the customer. The certifying authority (e.g. FAA/JAA) will have a maximum passenger count for the type. I'm reasonably certain that the BFE has to be certified as well, as you couldn't evacuate the type limit if the aisle wasn't in the center or you mixed 3-2 and 2-3.