How does Photos.app handle multiple libraries in relation to iCloud Photo Library? The new Photos.app allows you to hold down option to open up other libraries, for example Aperture and iPhoto libraries. 

How does iCloud Photo Library handle this? Does it work in a way like iTunes Match, where a 'central' library lives in the cloud, and various libraries add to it? Or will you have to overwrite the library if you open another. 

This question stems from another question I had on how to import an Aperture library after setting up a new one, but is more tied to how iCloud Photo Library handles this more than the Photos app - and the workflow/system around this.
Only one library can be the "system library", which in turn is used for iCloud Photo Library (if enabled), but you can have as many other libraries as you want.