This is not an easy question to answer. It was U10, then it became U13, and now it's U15. It is effectively developed on two bases. There is an attempt to be pan-Canadian in the representation of universities, and it also represents the universities that are most research-intensive. Take a look at those U15 universities. They effectively receive something on the order of 75% of all of the research funding in Canada, and they produce about 75% of the Ph.D. students. That's the simple basis. The fact is that U15 has mainly come together to try to share best practices between the universities in the research space.
On June 13th, 2017. See this statement in context.