Don't forget to sign it. This registration is good for your lifetime plus 50 years. It is standard practice to register your script with the WGAw. This registration is renewable for an additional 10 years. The source of your confusion is federal copyright statute.
Registration is a formality.
BUT, you do need to register in order to bring an infringement action in federal court. Which means, you fill out the registration form and pay the 20 bucks just before you file the papers to commence the suit. This is kind of a carrot to get you to register and file a couple of copies of your work with the Library of Congress. LC Marvel is the Library of Congress gopher server. Registration does not confer any statutory protection.