the only real difference is the boxing sanctions.  theres the WBF(world boxing association) WBC(World Boxing Council) IBF (International Boxing federation) IBO (International Boxing Organization) and a few more.  They're just different sanctions for the purpose of making more money.  But of course there could be a boxer who wins the nationaal championship in his country, and then travels to another country and challenges they're national champion and wins.  he'd be holding national championships from two different countries.  which would by definition make him an international champion.  Hope i was of some help to you.