The XHTML modularization model defines the modules of XHTML.
XHTML is a simple, but large language. XHTML CONTAINS most of the functionality a web DEVELOPER will need.
For some purposes XHTML is too large and complex, and for other purposes it is much too simple.
By splitting XHTML into modules, the W3C (World Wide web Consortium) has CREATED small and well-defined sets of XHTML elements that can be used separately for simple devices as well as combined with other XML standards into larger and more complex applications.