A post that is part of a larger set of the same image with minor changes between them. They are usually recognizable by a base image that is from the same unchanged perspective.
Common examples of changes within the set are:
For a post to qualify for this tag, it needs to have at least one parent or one child, or be part of a pool.
Note that two images that are identical but are from different sources (duplicate), simple censored/uncensored relations, and revisions are not variant sets. A visible change within the image is a requirement.
Not to be confused with variations, which are multiple variations within the same image.