On the page for the international application it says to use it if you’re ONLY applying to the University of Toronto. Do they mean if that’s the only Canadian school you’re applying to or like the only school you’re applying to at all?
basically, if you’re ONLY applying to uoft and you’ve never studied at a Canadian university before AND you live outside canada – you apply using this application.
if you’re also applying to other Canadian universities (in addition to uoft, that is), then you should apply using this form (the OUAC 105).
it’s just one of those finicky bureaucracy things, but make sure you use the right application, because it could cause you a lot of trouble later on if you make a mistake.