I believe Apple explicitly stated they were describing translucency and layers with iOS 7.

Both the Notification Center and the Control center on iOS "overlap" the screen through translucency. It could be argued that the parallax feature is also another attempt at differentiating layers within iOS, coupled with some of the animations i.e opening folders (also translucent), and multitasking.

That being said, I feel that iOS is unable to convey relationship of layers/overlapping windows well the way we are all accustomed to with desktop computers.