Using iCloud Photo Library with multiple libraries
Apple offers a service called iCloud Photo Library that allows you to easily sync the photos you have on your Mac with those you take on your iPhone, iPad, or even another Mac. Using iCloud photo library is optional, and is off by default - you can store your photos on your Mac without doing any syncing to iCloud if you wish.
Only one library can be synced with iCloud at any given time. You must first designate that library as the system photo library in Photos’ preferences, at which point you can enable iCloud photo library, also in Photos’ preferences. When viewing your libraries in PowerPhotos, it will mark your iCloud library with a label just beneath that library’s name.
There are currently not any options provided by Photos to sync a subset of your library with iCloud - it’s all or nothing. PowerPhotos can help with this limitation by allowing you to create separate libraries to store the photos that you don’t want to have synced with iCloud. For example, if you just want a relatively small selection of photos to sync with iCloud, you can set up a library just for that, then store the bulk of your photos in other libraries that are not synced. This helps save storage space used on your iCloud account.
Photos also provides an option to “optimize storage” of your iCloud library. When enabled, if your disk starts to run low on free space, it will not store the full size original versions of photos on your Mac, leaving them only on iCloud, and downloading them on demand as needed (e.g. when you view or edit a particular photo). When you have this option enabled, PowerPhotos will only be able to display photos that are fully downloaded on your Mac. Other photos in the library will not appear at all in the PowerPhotos image browser.
In addition to iCloud photo library, Photos offers two other way to transfer photos between devices: Photo Stream and iCloud Photo Sharing. These two features can be enabled or disabled independently of iCloud photo library, but like iCloud photo library, they can only be enabled for your system photo library.