

If this sounds confusing, then chances are good that the setting is not intended for you, so you should not change the setting at all, keep the default Importing choice enabled.

This contrasts with the default option of Importing (copying) the picture, where the Sample1.jpg image will not only stay in that original location but also be copied into ~/Photos/Photos Library.photoslibrary/ (or whatever the library is, if you make a new library in Photos it would be the active selected library). For example, if you have a picture file located at /Volumes/Backups/Images/Sample1.jpg, then Sample1.jpg will stay in that location and the Sample1.jpg file will not be copied into a Photos app library. This means the picture will stay in it’s origin location but basically have an alias in the Photos app (with thumbnails generated too, of course), rather than being copied into Photos Library files.

Instead, the only things stored in the Photos Library directory will be alterations to images, thumbnails of pictures, and iCloud data. With Importing turned off, you’re now free to add pictures to Photos app with the import function or drag and drop as usual, but the newly added images will no longer be copied to the Photos Library.

It is not recommended for most users to turn this off, Apple leaves this feature enabled by default for good reason.
