metadata.ExcludeClass

This on is real simple. "[ExcludeClass]" means that the class and its methods/properties/etc won't show up in intellisense. You can still import it, you can still use it, it's just harder to find. If you come across a class in the mx package marked with [ExcludeClass], then you're probably not supposed to use it directly. My personal guess is that these classes aren't stable and could change with any update.