Global Complex Types vs. Global Elements

Hi, all

When I read XMLSPY 2005 User Manual today, I found the following statement
which really baffles me. 

{{//http://www.altova.com/manual2005/XMLSpy/SpyEnterprise/globalelements.htm

A globally declared element can be reused at multiple locations. __?___It
differs from a globally declared complex type in that its content model
cannot be modified without also modifying the global element itself___?___.
If you change the content model of an element that references a global
element, then the content model of the global element will also be changed,
and, with it, the content model of all other elements that reference that
global element. 

}}

Since XML Schema Primer & Structures do not mention the above point
explicitly, could you give me some explanation as to the sentence?

Thanks!

Regards,
Collin