confused about extern use

I have seen a header file in which one structure is defined with
extern declaration in a header file and declared one variable of that
structure in one C-file. The code goes as mentioned below . I am
confused with the way it is declared and defined.

a.h
-------

:
:
:struct abc {
unsigned long a;
unsigned long b;
};
extern struct abc abc;
:
:

file.c
-------

:
#include <a.h>
:
:
struct abc abc;
:
:

Here in the above example I am confused about, what extra the coder
going to achieve by declaring it as extern in the header file a.h.

Thanks
Das

.

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