What does "supplemental" mean in the Google index?

If you search for some terms in Google, you might saw it: "Supplemental Result". This "tag" is going to vanish soon from the actual index, but the meaning of it remains:

An URL, which is indicated "supplemental" is not the first choice of Google to show you. It is supplemental, but not primary... That means Google decided, that the quality of that page might not be enough to be first choice, but yet good enough to mention.

There has been a lot of discussion about that topic and yesterday Google announced, that they will remove the indication from the public index. You can still find out, if pages are supplemental, but not directly from the results.

More information about that topic can be found in the Google official blog:

http://googlewebmastercentral.blogspot.com/2007/07/supplemental-goes-mai...

and more discussions are going on on webmasterworld.