iTunes Custom RSS Tags - Category
While the RSS 2.0 <category> tag may be populated with any text, Apple has a specific list of requirements for the <itunes:category> tag. Currently iTunes supports 19 categories, with a smaller subset of them supporting a specific list of subcategories. You may designate more than one subcategory where appropriate. These categorizations are used to determine the hierarchy within the iTunes Music Store Podcast Section, so it's important to pay attention to what is allowed. iTunes allows multiple top-level category definitions for the same podcast, but will default to the first one listed when cataloging the Podcast in the iTunes Music Store.
Within your RSS 2.0 feed, an iTunes category with two subcategories looks like this:
<itunes:category text="Computers" />
<itunes:category text="News" />
Top level categories are listed in bold below with subcategories immediately following. If a category has no subcategories, the next top level category is listed.
Arts & Entertainment
Movies & Television
Religion & Spirituality