Thread:Manual talk:Short URL/Apache/DOCUMENT ROOT/reply (3)

%{DOCUMENT_ROOT} isn't assigned in any file. It's natively handled by Apache itself. You won't see where it's handled unless you go looking at Apache's pre-compiled C source code.

I can't really help you figure out what file you should use, different operating system distributions put it in different locations. You should probably ask whoever setup the server for you or perhaps just use .htaccess files instead.