There is now no such thing, so do not worry about it.
In Google’s words:
“In 2003, Google introduced a “supplemental index” as a way of showing more documents to users. Most webmasters will probably snicker about that statement, since supplemental docs were famous for refreshing less often and showing up in search results less often. But the supplemental index served an important purpose: it stored unusual documents that we would search in more depth for harder or more esoteric queries.”
Webmasters used to be concerned that there was a problem if their site or pages from their site and would look for ways to get out of the supplemental index.
However, Google got rid of the supplemental index it in 2007:
“Now we’re coming to the next major milestone in the elimination of the artificial difference between indices: rather than searching some part of our index in more depth for obscure queries, we’re now searching the whole index for every query.”
Unfortunately some wannabe SEO’s missed the memo and still think its exists leading to a lot of confusion.