Understanding How ePublisher Works : Understanding Stationery, Projects, and Overrides : Extension Objects
 
Extension Objects
While extremely powerful, XSL has shortcomings when used to perform system-level scripting tasks. Some operations, such as working with files and advanced string operations, are not included in the standard XSL language.
XSL provides a generic extension mechanism to add new features to the base language. ePublisher provides a standard set of extension objects that enable XSL to generate all the required formats.
XSL extensions live in a specific namespace. You can define your own prefix to associate with a given namespace, but using a consistent naming convention makes life easier.