Since NotificationCenter is a part of Foundation. I expect it should be within RxSwift, not RxCocoa.
But why NotificationCenter+Rx is in RxCocoa? Any reason behind that?
In some of our project, we only use RxSwift without RxCocoa. If we use NotificationCenter.default.rx.notification, we will get the error Value of type 'Reactive<NotificationCenter>' has no member 'notification' because NotificationCenter+Rx is in RxCocoa only.
The easy answer is - RxSwift, RxJava etc are the platform-agnostic implementations of the ReactiveX.io standard, e.g. - Observables, Operators, Subjects, etc.
While the platform-specific things are, well, platform-specific ;-) In our case, RxCocoa covers NotificationCenter which isn't relevant to other, non-Cocoa platforms.