T1 has sterile int'l transit, T3 does not -- hence the difference in procedures, despite being the same airport. Very confusing when it varies by airline. T3 is changing to a new procedure this year, but nobody has confirmed whether it is sterile transit or merely priority immigration line (in any case an improvement!). For airlines using T3 (Oneworld and Skyteam) YYZ is a hassle compared to other airports or flying on Star (which is in T1).