icloudiosmacphotos
Photos for macOS has the ability to manually tag People in a photo where the machine learning algorithm has not yet detected a Face:
Window > Info > Add Faces > Click to Name
These tagged photos then show up when selecting a Person from the People Album in iCloud Photos, just as if they had been recognized by the ML algorithms.
How can I similarly manually tag People/Faces in a photo from the iOS Photos app?
The iOS 12 and macOS 10.14 versions of Photos makes this significantly easier.
Select the face / person / name in question and then select:
That purges the name and face from ones it will select in the People Album. I do see that both platforms will find names from your contacts, so if you just removed "Mary" from your album and you have any contacts with Mary in them, you will still get prompted. Temporarily deleting the contacts (make a backup) or deleting the sync of cloud contacts like iCloud and Outlook.com (or the many other contact sync options) you can test that Photos doesn't have that name anymore to suggest to you.
Drag anywhere in the circle to move the selection area & use the dot on the right to resize.
X on the left to cancel.
Best Answer
In iOS 14 and earlier, you cannot manually add a face tag on on iOS. This function is only available on macOS.