The <guid> element defines a unique identifier for the item.
Note: Aggregators must view the guid as a string. There are no rules for the syntax. It's up to the creator of the RSS document, to establish the uniqueness of the string.
Tip: GUID = Globally Unique Identifier.
|isPermaLink||Optional. If set to true, the reader may assume that it is a permalink to the item (a url that points to the full item described by the <item> element). The default value is true. If set to false, the guid may not be assumed to be a url|