During the introduction of sumplemental index pages by google on 2003, they labeled each index pages that under suplemental pages. Suplemental pages is like reserve index pages which are not priority in the search result for a target keyword. But they still play big rule in the search result, all suplemental index pages will show in the search result if no enough search result to show from google main index pages.

Late 2007, all pages labeled with Suplemental pages were suddenly gone, many webmasters and SEO experts thought that google have disregard this technolgy.

But everyone is wrong, they only remove the lable from all search result, but the technology is still there, they are still implementing the Suplemental index pages. If you’re using a duplicate content or very long url pages and hard to get your page or target keyword rank, mayby your page is labeled as Suplemental page.

If this happened, you may want to update the content of the said page and put more back link on it.