A Question on Module Implementation

In the section 1.3.2 of the REC "Modularization of XHTML" - Module Implementation - ,
1st paragrap says that "An attribute-list declaration in a module may modify an element type
outside the element types defined in the module, and a content model declaration may modify an 
element type outside the element type set of the module.".

I want to catch the meaning of "modify".