Supplemental results usually only show up in the search index after the normal results. They are a way for Google to extend their search database while also preventing questionable pages from getting massive exposure.
How does a page go supplemental?
From my experiences pages have typically went supplemental when they became isolated doorway type pages (lost their inbound link popularity) or if they are deemed to be duplicate content. For example, if Google indexes the www. version of your site and the non www. version of your site then likely most of one of those will be in supplemental results.
If you put a ton of DMOZ content and Wikipedia content on your site that sort of stuff may go supplemental as well. If too much of your site is considered to be useless or duplicate junk then Google may start trusting other portions of your site less.
Negative side effects of supplemental:
Since supplemental results are not trusted much and rarely rank they are not crawled often either. Since they are generally not trusted much and rarely crawled odds are pretty good that links from supplemental pages likely do not pull much - if any - weight in Google.
How to get out of Google Supplemental results?
If you were recently thrown into them the problem may be Google. You may just want to give it a wait, but also check to make sure you are not making errors like www vs non www, content management errors delivering the same content at multiple URLs (doing things like rotating product URLs), or too much duplicate content for other reasons (you may also want to check that nobody outside your domain is showing up in Google when you search for site:mysite.com and you can also look for duplicate content with Copyscape).
If you have pages that have been orphaned or if your site's authority has went down Google may not be crawling as deep through your site. If you have a section that needs more link popularity to get indexed don't be afraid to point link popularity at that section instead of trying to point more at the home page. If you add thousands and thousands of pages you may need more link popularity to get it all indexed.
After you solve the problem it still may take a while for many of the supplementals to go away. As long as the number of supplementals is not growing, your content is unique, and Google is ranking your site well across a broad set of keywords then supplementals are probably nothing big to worry about.
All of the text above has been copy&pasted from:
so, if those points are correct this post should go Supplemental in no time, right?
Wrong. Wait and see ...
Matt Cutts, a well known Google engineer, asked for feedback on the widespread supplemental indexing issue in this thread. As noted by Barry, in comment 195 Matt said:
Based on the specifics everyone has sent (thank you, by the way), I'm pretty sure what the issue is. I'll check with the crawl/indexing team to be sure though. Folks don't need to send any more emails unless they really want to. It may take a week or so to sort this out and be sure, but I do expect these pages to come back to the main index.
In the video below Matt Cutts answers questions about Supplemental Results,
Should I worry about results estimates for:
1) supplemental results
2) using the site: operator
3) with negated terms and
4) special syntax such as intitle: ?
Answer: No. That's pretty far off the beaten path
Getting Out of Google Supplemental Results
Getting out of the Google Supplemental Results may be possible by improving your website navigation system. To get more pages fully Google indexed, the prominence of important website pages can often be boosted by linking to them from pages within your domain having the highest Page Rank, such as your homepage. The reason for this being that Page Rank is passed from one page to another by links and the most common cause of Supplemental results is lack of Page Rank.
Start by determining your most important web pages which have been made supplemental - for example those promoting lucrative products and services, and then improve your website internal linking by adding links to these pages from more prominent fully Google indexed pages of your site including your homepage. At the same time, ensure that your website navigation system is search engine friendly using a website link analyzer.
By improving website navigation and getting more inbound links from other Worldwide Web sites, you may be able to get more website pages fully Google indexed, solving the problem of partial Google indexing and Supplemental pages.
Where the Google Page Rank of your website homepage is PR4 or PR3, improving your website navigation system and in particular the prominence of internal pages may help to get out of supplemental results. This can be done by including static hyperlinks from the homepage to your 'problem supplemental result pages'.
However, where your homepage is PR3 or lower and you have a large website, internal navigation improvements alone may still not be enough when it comes to getting out of the Google Supplemental Results. At PR3 or lower, your homepage Page Rank is probably too low to pass on enough Page Rank to your internal pages to completely get out of Supplemental Results.
To fully solve the problem of partial Google indexing, get more one way links to your site from quality web directories and sites of a similar theme and wait patiently to become fully Google indexed. In addition, getting more quality one way links pointing to internal pages of your website (rather than just targeting your homepage) is another powerful way of boosting the ranking of those pages against specific keyword terms, and it will also assist in getting them out of supplemental results. This is often referred to as "deep linking".
Google states that they have now removed the label "Supplemental Result" from their search result pages:
"Supplemental Results once enabled users to find results for queries beyond our main index. Because they were "supplemental," however, these URLs were not crawled and updated as frequently as URLs in our main index.
Google's technology has improved over time, and now we're able to crawl and index sites with greater frequency. With our entire web index fresher and more up to date, the "Supplemental Results" label outlived its usefulness."
Right, that is correct. It is called now "Omitted Results". Same thing really, and same side-effects, at least from this non-Google point of view.