Supplemental Results: Supplemental results are search engine results that don't rank in a search engine's main index. Instead, these pages appear in a secondary database which contains less trusted and lower-ranked documents. Google is an example of a search engine that contains multiple indices.
In Google, as with other search engines, a page's rank is typically determined by the quantity and quality of links pointing to it. Because Google devalues sites that use paid links, it may not display a page in its main results, but it could show it in another search instead. Although Google used to mark whether a site was part of the supplemental results or not, the search engine discontinued this practice in 2007, making both types of results indistinguishable from one another.
Many factors can contribute to a page's placement within the supplemental results. Using tactics such as posting duplicate content is one way to wind up outside of Google's main index. Pages with lower ranks, manipulative links, paid links, high individual page counts, and out-of-date content all have a risk of winding up in the supplemental index.
However, pages that don't fit into Google's main algorithm (or that of another search engine) can often improve their standing by using good SEO practices. One of these methods includes the use of quality backlinks -- meaning linking to that website by reputable pages only. Removing duplicate content and restructuring a site's links can also have positive effects on search rankings.