By usage or by tradition. The court has had occasion to talk about cabinet confidence. There was a case in the Supreme Court of Canada—I'm not sure what the year was; I think it was 1992, but I'm not sure. But it's tradition, practice.
If you go to the oath that persons entering the Privy Council, i.e., cabinet, take, that's an indication of what might be a cabinet confidence, where the person taking the oath says, “I shall keep secret all matters committed and revealed to me in this capacity”, as a member of Privy Council, “or that shall be secretly treated of in Council”. That's probably a good indication of what cabinet confidence is meant to cover: all those matters that are committed and revealed to the members of cabinet in the deliberations of cabinet.