Photos offers different options than iPhoto or Aperture for creating Smart Albums. Most Smart Albums from iPhoto or Aperture migrate to Photos with no changes. But if a Smart Album you created in iPhoto or Aperture uses conditions (Smart Album criteria) that Photos doesn't support, the Smart Album might look different or it might not appear in Photos.
Smart Albums that Photos displays
Existing Smart Albums from iPhoto or Aperture that use simple conditions appear similarly in Photos.
For example, an iPhoto Smart Album with images rated with two stars appears in Photos as a Smart Album with images with the keyword "2 Stars". An iPhoto Smart Album with images rated two stars or greater appears in Photos containing images with the keywords "2 Stars" or "3 Stars" or "4 Stars" or "5 Stars".
In both of these cases, the Smart Album displays the same images in Photos that it displayed in iPhoto or Aperture.
Smart Albums that Photos displays with modifications
When a Smart Album in iPhoto or Aperture uses conditions Photos supports and conditions it doesn't, Photos displays the album using only the conditions it supports.
For example, an iPhoto Smart Album with the conditions "My Rating is two stars" and "Place is Maui" appears in Photos with "(modified)" added to the name. This Smart Album contains images based solely on their star rating, since Photos doesn't support Place as an album condition.
Smart Albums that Photos doesn't display
If a Smart Album from iPhoto or Aperture uses only conditions that Photos doesn't support, Photos won't display that album. For example, an Aperture Smart Album that contains only photos edited in an external editor won't appear in Photos.