Example:
I have a site where members can sign up for sample chapters of our material from the main sales page.
The Signup Success page, basically thanks them for signing up and gives them a look at the first chapter.
However, I'd like to test a "splash" page that would have them "sign up" for access to the site at all - in other words, they wouldn't even see the sales page unless they provide their first name and email address.
Both forms would subscribe them to the same list, but the Sign Up Success page for the latter situation would basically be the Sales Letter, rather than the "Thank you" and first chapter.