This tag is used to indicate when a source does not directly point to the true source of an image. This includes redirect links, and links to a completely different image, but usually not non-normalized direct image links (as direct links typically don't redirect to different images or 404).
Such examples include but are not limited to:
Artist profile pages
If the source of the original image is found, please use it to replace the given source link (either as an image link or, ideally, as a post link).