The SET statement sets values of indexes associated with table
  elements.  It can also change the value of a conditional variable,
  change the status of an external switch, and store the address of a
  COBOL identifier reference at run time.

  Format 2 -

   SET { indx } ... { UP BY   } increm
                    { DOWN BY }

1  –  indx

  is an index-name.

2  –  increm

  is an integer, which may be signed.  It can also be the identifier of
  an elementary numeric data item described as an integer.