Simple List Extensions (SLE) is a set of extensions to RSS 2.0 and Atom feeds created by Microsoft to make it easier to exposing ordered lists through the feeds.
Simple List Extension contains a set of tags that tells the feed readers how to sort or filter the items in the feed. It also contains a tag that instructs the reader to interpret the feed as a list. If the feed is marked as a list the reader should treat the feed as the complete ordered list which means that readers must remove any cached items not present in the feed.
I did a test om my RSS feed with the extensions and when you watch it through a SLE enabled reader, Internet Explorer 7 for instance, you will now see the possibility to sort the items based on the number of comments each item has.
Note that when inserting the SLE tags the default Sort by and Filter by functions in IE7 is removed. I had to add the filter by category and sort by date and title to my feed.
This is how the feed looks like:
<rss version="2.0" xmlns:cf="http://www.microsoft.com/schemas/rss/core/2005"><channel>
...
<cf:listinfo><cf:sort element="pubDate" label="Date" data-type="date" default="true"/><cf:sort element="comments" label="Comments" ns="http://purl.org/rss/1.0/modules/slash/" data-type="number"/><cf:sort element="title" label="Title" data-type="text"/><cf:group element="category" label="Category/tag"/></cf:listinfo>
...
</channel>