What is the Atom "feed/subtitle" element? How To Generate a "feed/subtitle" element?

The <subtitle> element is not a required sub-element of the <feed> element. But it is strongly recommended that you provide a subtitle element for each of your feed documents. You should follow the same rules on Web page description meta elements to generate your Atom feed subtitle elements.

One strategy is to use the same value of the description meta element as the Web page that your Atom feed is based on. For example, if your Web site provides a Web page for the most popular FAQ entries for Webmasters. That Web page has the following description meta element:

 <meta name="description" content="The top 10 popular 
  entries of this week on our comprehensive collection 
  of Webmaster FAQs."/> 

Now you want to generate an Atom feed for that Web page. Obviously, you should borrow value from the description meta element of that page:

 <title>The Most Popular FAQ Entries for Webmasters</title>
 <subtitle>The top 10 popular 
  entries of this week on our comprehensive collection 
  of Webmaster FAQs."</subtitle>

⇒Atom Specification - RFC 4287

⇒⇒RSS FAQ and Tutorials