What if that manager needed to get a value from a proxy ?Then it is probably better written as a Mediator who communicates with the system manager rather than a normal component. The system manager can be thought of as a view component, even though it's not part of the view hierarchy.
I find this interesting and confusing at the same time.
I've always approached Mediators as requiring a viewComponent instance to mediate. But reading the above, it seems the Mediator class can be thought of as a sort of catch-all class for doing anything the Proxy or Command isn't clearly supposed to.
So for example, in my case, where I use a Command to access the PopUpManager and have it .addPopUp() a view component passed via the notification.getBody(), I should really have a PopUpManagerMediator class??? (even though the PopUpManagerMediator would not be shepherding any specific view component)