question about ZMAT in GAMESS-US

Dear Everyone:
              Here is a question about the $ZMAT in gamess-us. In the
 input.txt, there is a example about how to set the IZMAT, but I do not
 understand what it means. It seems that it is different from the Zmatrix in the
 $data field. I wish you could give me some detailed explanation about it. Thanks
 in advance.
 a nonliner triatomic(atom 2 in the middle), The IZMAT(1)=1,1,2   2,1,2,3  1,2,3.
 Best Regards,