OK, what you are saying (if I understand correctly) is that a "sold out" train simply has no more RESERVED seats available. But you can line up on the platform for seats in the UNRESERVED cars, and any seats that may happen to be available in those are assigned by your position in line.
This is good to know. It's unlike any other trains I've traveled on (in Europe or North America), where there are unreserved seats you just have to search for, walking through car after car while schlepping your luggage.
Seems like Japan has a very good, well-organized system!
Does this apply only to the high-speed trains?
Kathy