System Photo Library overview for Photos on Mac
When you first use Photos, you create a new library or select the library that you want to use. This library automatically becomes your System Photo Library. Although you can use multiple photo libraries in Photos, the System Photo Library is the only library that can access iCloud services, including iCloud Photos, Shared Albums, and My Photo Stream. In addition, some Apple apps and third-party apps also access your System Photo Library using the Media Browser.
By default, your System Photo Library is stored in the Pictures folder on your Mac, but you can move it to another location on your Mac or store it on an external storage device.
WARNING: If a Photos library is located on an external drive, don’t use Time Machine to store a backup on that external drive. The permissions for your Photos library may conflict with those for the Time Machine backup.
If you want, you can create additional photo libraries and store them in any location on your Mac or on an external storage device. However, keep these things in mind:
You can work with the photos in only one library at a time. To work with photos in another library, you must switch between photo libraries.
If you switch to a library other than the System Photo Library, the changes you make are not reflected in iCloud Photos, because iCloud Photos only syncs changes from your System Photo Library. If you create any other libraries, make sure they are backed up using Time Machine.
By default, all the files you import into Photos are stored in the current photo library. If you want, you can set Photos to leave the files you import in their original locations (for example, on a storage device) and not place them in the library. See Change where Photos stores your files.
WARNING: To avoid accidentally deleting or corrupting a Photos library, do not alter the contents of a library in the Finder.