Referring to the little bar under each thumbnail in a post list; the one that has the score, favcount, comment count, rating, and mystery letter.

So far, the values I have seen for mystery letter are "U", "C", and "". I had initially thought that it might be the status ("unconfirmed", "confirmed"), but some thumbnails lack a mystery letter altogether (""), which throws a wrench in that line of reasoning. Also, the term is 'approved', not 'confirmed'.

I can't seem to find anything with a search, either. The closest match was about border colors, where someone linked to the help page on posts. Said help page does not mention these thumbnail statuses.

Parent, child, unapproved, flagged