When an image is Watermarked by a third-party website such as an image gallery, wallpaper site, or an anime or gaming website displaying Official Art.
Third-party watermarks are disallowed on Danbooru. They are regarded as flaws that should be addressed by someone finding a watermark-free image and uploading it. Once one is uploaded, the image with the third-party watermark should be made a child post and flagged.
Sites such as Weibo which add the artist's username as a watermark should not be tagged third-party watermark except in cases of Third-Party Sources.