Globally Unique Identifier - XML Syndication Formats

XML Syndication Formats

There is also a guid element in some versions of the RSS specification, and a mandatory id element in Atom, which should contain a unique identifier for each individual article or weblog post. In RSS the contents of the GUID can be any text, and in practice is typically a copy of the article URL. Atoms' IDs need to be valid URIs (usually URLs pointing to the entry, or URNs containing any other unique identifier).