My question is if I have say 13gb of photos in iCloud photo library and on my iOS device I have say 6Gb of free storage. Does the photo's get downloaded and when I run out of space on the iOS device the photos will stop syncing with the device? Or do the photo's stay in iCloud photo library and don't use the device storage? As you can see, it will be easy for some people to run out of space on their iOS devices if they have a lot of photos.