Supplemental index was introduced in 2003, to differentiate it from the rest of the results. The pages which falls in to supplemental index seldom ranks, shown rarely in SERPs.
Supplemental pages made the webmasters/SEO understand that they need attention to get them out of the supplemental index and to gain a place in the index document. Google clearly labelled the supplemental pages, once necessary work is done to those pages to bring them out of the supplemental pages, google removes the label and keep them in normal results.
Google came up with a major update on July 31 2007 to remove supplemental label and to unite it with normal results. Now webmasters/SEO confront the issue of finding out which are the supplementary pages, eventhough the label has been removed the qualities and ranking nature remain the same for supplemental pages.
On Dec 18 2007 google has assured justice for supplemental results by eliminating the distinction between "supplemental pages" and "normal pages."
Find below the comments from google:
Now we're coming to the next major milestone in the elimination of the artificial difference between indices: rather than searching some part of our index in more depth for obscure queries, we're now searching the whole index for every query.
From a user perspective, this means that you'll be seeing more relevant documents and a much deeper slice of the web, especially for non-English queries. For webmasters, this means that good-quality pages that were less visible in our index are more likely to come up for queries.