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.
Best Answer
I was under the impression, that this was just to improve visibility of the white font, but since you're saying the bands come and go, that's probably not it. I'm on iOS 11 Public Beta 6.