B Commerce (International) - compulsory courses required out of the liberal studies stream. Compulsory 1 year exchange to another country.
B Commerce/B Arts - B arts component - choose at least 1 arts major and 1 arts minor. So you're going to be specializing in more fields. Ultimately, you can do double arts major if you want to, but you would either need to overload, take summer school for commerce courses or stay an extra year.
So for both programs, you can major in International relations. It just that the former has more compulsory courses, and the latter is more flexible.