Details of how Photos handles the migration from iPhoto
How Photos handles content and metadata from iPhoto and Aperture - Apple Support
How iPhoto content and metadata migrate to Photos
Photos and organization:
- Photos and movies migrate without changes. Your images migrate with the adjustments and filters that you applied in iPhoto. You can't change the adjustments in Photos, but you can revert to the original image.
- Events migrate to a folder that's labeled "iPhoto Events" in the Albums view.
- Photos preserves albums.
- Photos preserves most Smart Albums. Smart Albums with selection criteria that Photos doesn't support usually migrate to Smart Albums with "(modified)" added to the Smart Album name. If Photos doesn't support any of the Smart Album criteria, it doesn't migrate the Smart Album.
- Some photo books and calendars, made with themes that Photos no longer supports, migrate to Albums.
- Photos preserves slideshows. If a slideshow theme isn't available in Photos, it uses the default Photos slideshow theme.
- Photos preserves Faces data.
Keywords and other metadata:
- Photos preserves keywords.
- Photos preserves user-defined titles. Images without user-defined titles show as "untitled" in Photos.
- Star ratings migrate as similar keywords in Photos, such as "1 Star," "2 Stars," and so on.
- Flagged images migrate with the keyword "Flagged" and appear in the "Flagged" Smart Album in the Albums view.
- Photos preserves most metadata. Some IPTC (International Press Telecommunications Council) metadata, including Copyright, won't appear in Photos. This metadata is still associated with the image, and you'll see it in applications that show IPTC metadata, like iPhoto and Aperture
How Photos save disk space using Hard links
Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support