The supplemental index has become a tool for Google to place less valued pages into to try and fine-tune the quality of its main index. In fact, the current supplemental index is growing rapidly and may be trying to get into the “Size Wars” of Millions of pages indexed. – OK, so I may like to exaggerate a little, but the fact is this is an index that is going through changes and has its own updates. Meaning 6 months ago a site in the Google index was there for duplicate content, now there are other reasons that Google is using to value sites and place poorly created pages into the supplemental index.
The current version is very simple and easy for many sites to get into, particularly sites that are smaller retailers than the trusted Authority domain retailers like ebay and Amazon.
You get there with weak content and weak inbound link quality, or no inbound links internally and externally. Sounds tricky? Well, its really quite simple its a low quality automated factor, meaning a robot is judging things like links into the page, content and how it differs from other pages, and the value of importance the internal navigation places on the page.
Make sure there is a way to get to each of the internal pages, a sitemap used to be enough, but the sitemap is only one way, the more ways we can get to the internal pages the better.
A link on the home page makes a page important in the search engines eyes. A link from all pages makes the page a main page in Google’s eyes.
So when you are navigating through sections use breadcrumbs and try to get “specials” and “most popular” features to link into the products that have your highest return.
If Google thinks a page is of little importance on your site due to your navigation, lack of external inbound links and content that is almost identical to every other pages, then it will value it accordingly. So be sure you value your pages properly and set up an underlying architecture that will be strong.