%b between %{} in default $index_format

[Scott McDermott]

What possible use is "filename of the original message folder" between
`%{}'? IOW, what relevance does the message date have to the filename of
the message folder? The default $index_format is:

        %4C %Z %{%b %d} %-15.15L (%4l) %s

And I'm trying to understand why `%b' is between those braces, when date
would not affect its output...

Incidentally, said expansion does not seem to occur anyways (the `%b')
at least in my index.  There is no filename...huh?

-- 
Scott