theres not a difference really, international open 5 is ages 14+ and international open 6 is 18+. those are the divisions that most teams from other countrys compete in at worlds. but up untill then the ioc teams from the us usually compete against the us.
Also, in the international divisions only 3 teams per country make it to finals. In the regular divisions it doesnt matter what country you're from.....it's all by scores. That's why most teams out of the U.S. compete in international divisions (they have a much better chance of going to finals).