Another useful application model involves request includes. In this model, the request
reaches an initial JSP page. The page may start generating/composing some result but at
some point it may want to dynamically include the contents of some other page. These
contents may be static but may also be dynamically generated by some other JSP page,
Servlet class, or some legacy mechanism like ASP.
Although in some cases this inclusion model is applicable to presentation dependent
contents, it is most often used in the context of a presentation independent content, like when
the data generated is actually XML (which may be converted later into some other format
using, say, XSL).
JavaServer Pages 1.1 Specification November 30, 1999