… sites [that] are part of Google’s auxiliary index. 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. For example, the number of parameters in a URL might exclude a site from being crawled for inclusion in our main index; however, it could still be crawled and added to our supplemental index.

The index in which a site is included is completely automated; there’s no way for you to select or change the index in which your site appears. Please be assured that the index in which a site is included does not affect its PageRank.

When your blog / website contents are crawled for supplemental index it means that they will never come up for primary or normal index for long time unless and until you submit consideration request again. It will come in a normal SERP (search engine result page) if there are no result are found in other website with a particular keyword.

Google supplemental result is just like Google Sandbox. The difference is the latter name was not officially coined by Google while the former was.

Things than can make your blog / website being supplemented by Google including, but not limited to:

• Paid linking
• Duplicate contents
• Spam
• No inbound links from a good site (with good Google PR).