Goodness, this all murky. Part of the problem is the use of formal, informal, and casual. Most people interpret informal as a notch above casual. Casual being daytime touring clothes (t shirts, shorts, hiking pants), informal meaning shirt and long slacks for men, formal meaning coat and tie - and of course, slacks 😎. SS has their own definition which we will respect but the murkiness is evident in that the rules only apply to classic voyages. And, from reports, even classic voyages have random enforcement.