You may have enough storage on your Mac, but don’t want to scroll though tens of thousands of images on your mobile devices. Referenced files do not have to always be available to be accessible, and Photos for macOS just stores a thumbnail and a pointer to the original. Use referenced files to keep tens or hundreds of gigabytes on an external drive. You have a large set of photos and not enough internal storage. You might find this useful for several reasons: Referenced files have a special overlaid badge (highlighted in red).

With the box unchecked, import images, and those that are added will get an extra badge overlaid that indicates the media file is referenced and not part of the Photos library. With iCloud Photos enabled, some smaller text makes sure you’re clear that items imported with this unchecked won’t be uploaded to iCloud Photos and, if you uncheck it, you also see a dialog box with the same message that you have to click OK to dismiss. In Photos > Preferences > General, you can check or uncheck the box next to Importing: Copy Items to the Photos Library.

How to link ipad and iphone photos