The source namespace is a collection of add-ons to RSS 2.0 that provide information about where the content in the feed comes from, so it's possible that aggregators can provide a richer experience for readers. It's the format that Fargo generates starting in version 1.64.
The example above is very simple and contains no nested structure, which is what makes outlines useful. Check out the Scripting News feed for more extensive examples.
Each of those folders contains one folder for each month, 01, 02, 03, 04, 05, 06, 07, 08, 09, 10, 11, 12. The month folders must be zero-padded to two places. Folders may be missing, indicating that there is no archived content for the month.
Each day folder contains a file named rss.xml, unless the optional <source:filename> sub-element is supplied, which overrides the default. This makes it possible for a single calendar structure to store the archive of more than one feed.
<source:archive> must contain at least two sub-elements: <source:link>, the address of the archive and <source:startDay>, a hyphen-delimited date, formatted as yyyy-mm-dd, for the first element of the archive.
A simple entirely human-readable way for the editor of the site to see what time, in his or her time zone, the feed was last updated. Must-have for debugging and sanity-preservation if you have trouble converting GMT to local time in your head (as I do). The format here is entirely up to the editor of the site.