This post is a bit esoteric, but worth understanding. Many times an author may not register a work when first published, but as times go on, and get better, it may occur to them to register the work. By then there may have been significant additional authorship added to the work. You might think you have to go back, find each iteration of your creation, and file each as a separate work in order to get the benefits of copyright registration. And that might be the ideal. But it might also be time consuming and expensive, or impossible. Under some circumstances you can get a good result by just filing a application for the derivative work. There is case law in the 9th Circuit (and others) that says you can at least get standing to sue on ALL PROTECTED MATERIAL in the derivative, not just the new material. Here's the rub: the older preexisting material must be in the derivative work, and the owner of the derivative must be the owner of the original work. The case is Religious Technology Center v. Netcom On-Line Communication Services, Inc. 923 F.Supp 1231, 1241-1242 (N.D. Cal. 1995).
Questions and comments welcome. Have a great day.