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A post whose source is plain text, rather than a link to the website where it originally came from.
This includes file uploads, where the source is simply the filename used by the uploader, and posts where the source is a written description of where the image came from. Written descriptions are often used for self-scans, self-uploads, commissioner uploads, and self-datamined game assets. These things have no source link because they come from an offline source rather than another website.
Non-web sources should be avoided whenever possible. If you obtained the image from a website, you should post a link to the website where you obtained the image. Do not use a written description or filename for anything you obtained from another website, including scans and game assets.
If you see this warning while uploading, it means that on the upload page you should paste the link to where you found the file, rather than uploading the file from your computer. Danbooru supports uploading from most sites, so in most cases it's not necessary to download a file to your computer first before uploading it to Danbooru. If you don't remember where it came from, do a reverse-image search first to try to find the source. See help:upload for details.
You can search non-web_source source:file://* to find file uploads, or non-web_source -source:file://* to find posts with written descriptions.
This tag is added automatically. Posts with no source at all aren't included under this tag.