Stumbled upon the RSS on Microsoft Watch, which included instructions for using USM with RSS. USM allows you to automatically subscribe to the RSS simply by clicking on the RSS link.
Previously, the RSS links to the XML file when you click on it. It is required to copy the link and register it with your favourite RSS reader. With this mechanism, the instruction to register the RSS is embedded within the click using a specific MIME-type, which can be handled by a USM application. The application would interpret the RSS URL and register the feed with your favourite RSS reader that the USM application supports.
The protocol is fully specified at this location: