What is "supplemental" in Google

[rfontaine]

When pages on your site goe "supplemental" in Google, does the entire site go supplemental or only certain pages?

And what causes a site or particular page to go supplemental?

[watercrazed]

Individual page go supplemental but ALL the pages in a site may go supplemental.

[strangerrr]

Supplemental results in Google are from an alternate index. Google usually only uses this index if they cannot find relevant results in their normal index. That means that this index is only used for very obscure queries.

Generally I see the reason for pages to go Supplemental is 404 page, old page on server (not up to date), dupplicate content pages... etc.

Also you can find lots of discussion here on Supplemental Result...
[google.com...]