[graph] named vertices?

I notice that in the changes for the newest boost that "named vertexes"
is now supported in adjacency_list.  I can't find anything on this
feature though.  I have the book, have searched the web.  Only thing I
find is comments in adjacency_list.hpp that maybe refers to a NamedGraph
concept?  I don't see that in the docs either:

http://www.boost.org/doc/libs/1_39_0/libs/graph/doc/table_of_contents.html

Could someone give me an overview of what this feature is and why I
might use it?  Probably better...point me to some place that documents it?

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