From http://www.w3schools.com (Copyright Refsnes Data)

RSS <cloud> Element

Definition and Usage

The <cloud> element is used to register processes with a cloud to be notified immediately of updates of the channel.

Tips and Notes

Note: Clouds can be implemented in either xml-rpc or soap.

Example

<?xml version="1.0" encoding="ISO-8859-1" ?>
<rss version="2.0">

<channel>
  <title>W3Schools Home Page</title>
  <link>http://www.w3schools.com</link>
  <description>Free web building tutorials</description>
  <cloud domain="www.w3schools.com" port="80" path="/RPC"
  registerProcedure="NotifyMe" protocol="xml-rpc" />
    <item>
    <title>RSS Tutorial</title>
    <link>http://www.w3schools.com/rss</link>
    <description>New RSS tutorial on W3Schools</description>
  </item>
</channel>

</rss>

Complete RSS Reference