module declaration and compilation

HI, I am not sure if this is the right place, but I have a question
about the module declaration and the compilation.
The proxy module and the mod_info module are both the only modules that
have a declartion with "extern" :
 
extern module MODULE_VAR_EXPORT proxy_module;
 
All others module I have seen have a module declaration without the
"extern", like 
module MODULE_VAR_EXPORT example_module;
 
Could someone explain the difference about these two declarations for
compilation and programs running.
 
Thanks
 
 
Gael Seroul