"Top 6" most often includes the listed prerequisite courses (but not recommended courses, or if they require, say, 2 of bio, chem, and physics and you have all 3, it would be the best 2 included) plus some to round out 6 courses. Some places just look at the straight top 6.
"Individual selection from the mid-80's", which is the sort of wording Waterloo likes to use, reflects on the fact that they aren't looking just at marks. So lets say for Engineering, they might take Joe with an 84, with his better marks in math, who did well on the Math contest, and who has a good recommendation, over Bob with an 86 whose best mark is in English and who didn't do so well on the Math contest - if you've got mid 80's, it's going to depend on what the rest of your application is like. I think with Waterloo, the closest you are to that low end, the more important the supplementary is.
Remember that it often takes just as long to do a search or two on Google to try and find the answer to your question as it takes to type it in a post here, let alone the time waiting for a reply.
If your question is "What mark does it take to get into University X", or "What are the requirements to get into University X", it's on their website and/or
electronicinfo.ca (for Ontario schools). Particularly for Ontario schools, those ranges are a pretty good picture of what kind of mark will get you into the program, and if you're more than a couple percent higher it's practically guaranteed unless it requires a supplementary form.
