However you can indeed use it from Safe Rust, and you can choose Orderings which do not give you Sequential Consistency if you're comfortable with that, or more likely because you are implementing an algorithm whose authors assure you this is fine (I hope they're correct). I think that's a very modest caveat to the overall promise of Sequential Consistency, but sure, it is a caveat. If you write code that explicitly asks for Ordering::Relaxed and are astonished you don't get Ordering::SeqCst I guess I apologise for confusing you.