[Dovecot] understanding namespace configuration

[btb at bitrate.net]

i'm a little confused about namespace configuration.  reading http://wiki2.dovecot.org/Namespaces , and looking at the examples, it seems that when a namespace is declared, you can either do "namespace { ..." or "namespace foo { ..." - i'm having trouble finding clarity on the significance of "foo" though.  it would appear it's not required, but then what is its purpose?  what does it mean?  does it imply something?

it's also unclear to me how to create the "default private namespace".  i see that there are three types of namespaces, but the list of namespace settings doesn't seem to include a parameter to set the namespace type.  what am i missing?

it's also not quite clear how to denote a namespace as "default".  from coming across this thread [ http://mailman2.u.washington.edu/pipermail/imap-protocol/2010-May/001077.html ] it would indicate in theory that a namespace is denoted as default by using an empty prefix - but i wasn't able to find corroboration of that in dovecot's documentation.

thanks
-ben