What is the Google Supplemental index?

When Google originally introduced Supplemental Results in 2003, our main web index had billions of web pages. The supplemental index made it possible to index even more web pages and, just like our main web index, make this content available when generating relevant search results for user queries. This was especially useful for queries that did not return many results from the main web index, and for these the supplemental index allowed us to query even more web pages. The fewer constraints we're able to place on sites we crawl for the supplemental index means that web pages that are not in the main web index could be included in the supplemental.

Link:
http://.../supplemental-goes-mainstream.html