This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
You can look at the XML DTD and determine if it is required or not.
The purpose of the tag is to allow the user to create "packages" out of Struts web contexts, similar to how Java packages work. See this article.