User blog comment:Rupert Giles/Layout Changes: Breakpoints and Typography/@comment-4130826-20150518202641/@comment-452-20150518211937

Who says you can't use it sitewide?

Adding custom modules to the right rail is allowed so long as the top ad is not displaced. Staff have said this multiple times, including Thread:720672 and Thread:814039

edit: Yeah, it can get hard to keep track of what is and isn't allowed, and why.