I'm working with macOS High Sierra beta 17A352a, Photos 3.0 (3201.11.120). I don't know if this works similarly in earlier versions.
When looking at people in the app, most portraits shown simply provide a key photo of the person with their name at the bottom.
Some portraits, however, show a grey band over the person's name.
What does the band, or lack of, indicate? I think it means that that person is having some processing done, because leaving the People section and returning usually has the bands gone.