Carousel is essentially watchOS's graphical user interface (much the same as GNOME is to Linux, Explorer is to Microsoft Windows, and Finder is to Mac OS) and manages graphical services such as icons and multitasking. When it is launched, it registers PurpleSystemEventPort, essentially making it the main application. Perhaps you already know about SpringBoard on iOS, Carousel is the equivalent to it for watchOS devices.
- Carousel acts a stacking window manager, rendering the child applications
- Carousel activates/deactivates and manages memory of the UIKit applications it launches