First, it's a lottery. You'd either get on the guaranteed list or the waiting list. If you're on the guaranteed list, you'll have to maintain a sessional average of at least 5.0 in order to remain on that list. If you're on the waiting list...you'll just have to cross your fingers! (but all my friends who were on the waiting list last year and the year before got a spot on res in the end.)
Upper years who have a sessional average of 9.5+ will be automatically guaranteed a spot on res, and I'm pretty sure that the rest of the upper years are NOT ranked by marks. It's just...as long as you have at least a 5.0. Your place on the guaranteed list may be higher or lower than others...but it's all random. The ordering is not based on marks.
Unless things have changed for next year...but I double checked the
Res Admissions website and the process remains the same.
Hope that helps!