[Evolution] What is the Search Folder "Unmatched" for?

[Jonathan Ryshpan <jonrysh pacbell net>]

Other Search Folders can be provoked into producing their "Properties",
including the search criteria.  The "Unmatched" folder produces a very
uninformative error box.  What is this folder for?  What criteria must a
message fulfill to get into it?  A web search has been uninformative.

Thanks - jon