Supplemental result pages are generally pages that Google has determined to be secondary to other more relevant pages that Google has indexed on your website. These results usually only show up in the search index after the normal results. It’s a way for Google to extend their search database while also preventing questionable pages from getting massive exposure through the search results. For example pages may be orphans, doorway pages with no inbound links, empty pages or have content that Google cannot index . SERPS from the supplemental index are shown only where there are very few matches shown from the main index.

There is nothing particularly bad about supplemental results but due to their position in SERPs they generate very little or no traffic. the result may eventually disappear from the SERPs. Since supplemental results are not trusted much and rarely rank they are not crawled often either and are generally not trusted much and it is also likely that links from supplemental pages do not pull much weight in Google.

Supplemental updates are infrequent and results can stick around for up to a year. If a large proportion of a site’s results are supplemental, sometimes the whole site can go supplemental except for the home page, which would affect your site’s optimization efforts and traffic badly.

Supplemental results are also result of Google finding near duplicate pages in terms of meta-data, content or URL. Duplicate content means that you are not wholly in control of this process and it will dilute any optimizations that you make and may result in parts of your site not being properly indexed. The usual causes resulting into duplicate content are multiple hostnames and database driven dynamic websites where different URLs can refer to the same page.

The best way to avoid supplemental pages is by ensuring that every page within your site uses unique content and provides excellent and relevant information. This way you will have a better chance of having Google determine that your pages are second to none.