

If you have more than one, the first photo library that you create or open in Photos will become the System Photo Library. If you have only one photo library, as most folks do, it’s the System Photo Library. Plus, some Apple apps and third-party apps also access your System Photo Library using the Media Browser. You can also sync them to iOS devices and Apple TV via iTunes. If you want to use your own images as your desktop picture or screen saver, the images need to be in your System Photo Library before you can select them in System Preferences.

You can create multiple photo libraries on your Mac, but you have to choose one to be THE System Photo Library. Photos in it are available in apps like iMovie, Pages, and Keynote.

You’ll need to properly manage them, including taking steps to make sure you don’t run out of local storage. When you first use Photos, you create a new library or select the library that you want to use.
