An image sample from Fandom. These are identifiable by the /scale-to-width-down portion of the source URL.
Posts potentially missing this tag can be found with the source:*/scale-to-width* -fandom_sample search. Note that Danbooru automatically resolves direct uploads from Fandom with this part of the URL included to upload the full size, even if the source field says otherwise, and sometimes the width it's scaled down to is the same as the image's full size.
