Supplemental sites are part of Google’s auxiliary index. They are also called "Google Hell", and "Screwed Pages" .
We’re able to place fewer restraints on sites that we crawl for this supplemental index than we do on sites that are crawled for our main index.
Why does google put pages in the Supplemental Results?
There are mainly 3 reasons of supplementary pages:
1. Duplicate Content - take someone elses content, get sent to Google Hell.
2. No Content - create pages with no content - empty pages get sent to Google Hell.
3. Orphaned web pages. Pages that no one links to, including yourself.
Now, How to solve this problem?
1. If you stole content - change it.
2. If there’s no content - add some.
3. If it’s orphaned - link to it.
You can find the webpages which are supplementary index by command
site:www.yourwebsite.com *** -view
Remember: There must be a space before and after ***
The Reasons of why you should add more content to your website?
->Your homepage has lots of backlinks, but few words on it.(You’re loosing out on your most powerful page. It’s really hard to rank for phrases that don’t appear on the page.)
->You’ve got 50,000 pages, but only 100 have more than 50 words on them .
->You’ve got the same content on your pages as 50 other sites have.(Google will try to choose the 1 page it thinks is the "real" one, will it be yours? or will yours go supplemental?)
->You’ve got hundreds or thousands of product pages that only have a small description.(It’s hard enough to keep these from going supplemental, let alone rank for anything with little content).
->There’s nothing on your site that’s "link worthy" from your peers, or from anyone else.(Having quality researched content can garner you natural backlinks.
->You’ve been targeting 3-4 "big phrases", but have been ignoring all the other phrases.(Having content pages covering lots of 3 and 4 word keyword phrases helps pick up related long tail phrases. Also, focusing only on a few "major phrases" isn’t good "insurance", and is getting harder).
->Google tries to rank "resource pages" high, and "resource pages" tend to have lots of original content.
Accoording to Matt Cutts
Total Pages Indexed = site:www.yoursite.com
Pages in the Main Index = site:www.yoursite.com -inallurl:www.yoursite.com (or allinurl:www.yoursite.com)
Pages in Supplemental Index = Total Pages Indexed - Pages in the Main Index
