If you do a site:domain.com search for your domain, you might see pages that are labeled as “Supplemental Result.” This means these pages are in Google’s supplemental index. In general, this index is supplementary to Google’s main index because pages from the main index usually appear before pages from the supplementary index for search engine queries. One way or another, these pages haven’t gained trust. A page might find itself in the supplemental index if:

• It is an island. i.e. No other pages in your site or other sites link to it.
• It has substantial duplicate content to a more authoritative page.
• It has very little content at all.
• It is deeper in your site and doesn’t have outside links to it.
• It hasn’t had certain filters applied to it yet.
• It is being redirected.

Of course these observations are officially unverified, but it’s a good guide to what might get your pages in the supplemental index. SEOs also speculate that PageRank factors are used to determine how many pages a site might have in the main index of Google.

Pages in the supplemental index are spidered less frequently. They should be thought of as lower quality than main index pages. If you have important pages in the supplemental index, the best way to get them out is by getting good links to them. You might also revise them so that they have unique or substantial content, if they don’t already.