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

RSS <category> Element

Definition and Usage

The <category> element specifies one or more categories the channel or item belongs to. The <category> element can also specify the hierarchic location in a category (the value of the <category> element is then a forward-slash separated string).

The <category> element makes it possible for RSS aggregators to group sites/articles based on category.

Attributes

Tag	Description
domain	Optional. A string or URL that identifies a categorization taxonomy. Example: <category domain="syndic8">IT</category>

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>
  <category>IT/Internet/Web development</category>
  <item>
    <title>RSS Tutorial</title>
    <link>http://www.w3schools.com/rss</link>
    <description>New RSS tutorial on W3Schools</description>
    <category>News</category>
    <category>Tutorial</category>
  </item>
</channel>

</rss>

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