At the end of the day, it's a decision for the Governor in Council. The Governor in Council makes the determination under the act as to whether there has been a “grave breach”. “Grave breach” isn't defined in the act itself. It really is a question of the context. Advice will be given to ministers. Ministers will then take that into consideration, make the determination, and make a recommendation to the Governor in Council. That's the process, and in a way that's the substance, too.
On October 17th, 2016. See this statement in context.