Keywords: A tag icon overlays the image which, if carefully clicked, reveals associated keywords. However, the keywords can't be clicked. To search by keyword, you have to enter it in the Search field in Photos' upper-right corner.
Edited: Edited images appear with a toolbox overlaying their lower right.
File Type: For videos, slo-mo, and time-lapse clips, an appropriate icon is overlaid.
Favorite: This reveals whether you've marked an image as a favorite and also lets you click while hovering over it to give it some love.
Referenced File: Images that aren't stored in the Photos library but elsewhere have a square-plus-arrow icon. Choose Show Referenced Image in Finder by Control-clicking the image or, with the image selected, choose the item from the File menu.
A caption or "description" doesn't appear in any view, but will appear in the Info pane when the image is selected.
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