This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
"extends" is not one of the standard validation rules that ships with struts. It looks like someone working on this application before you has written a custom validation rule called "extends". If you look in the validation-rules.xml file, you should be able to see more information about it.