The Links box in the edit page of a post contains the list of Suggested References associated with the post you’re editing:
The circle that appears on the left side of the reference indicates the status of the reference itself. If the circle is empty, the reference has not yet been included in the text of the post.
On the other hand, a full circle indicates that the reference has already been used in the text of the post. In addition, reference information appears in a lighter color to indicate that the reference has already been used. Note that Nelio Content automatically detects the usage of the suggested references and updates the status of these links in the box.
Finally, if Nelio Content detects that some of the suggested references is a broken link (the content no longer exists), the reference is marked in red to warn the author or publisher.