Indexing is a special subscripting procedure.  In indexing, you use the
  INDEXED BY phrase of the OCCURS clause to assign an index-name to each
  table level.  You then refer to a table element using the index-name as
  a subscript.

  Format -

   { data-name      } ( { , index-name [ { + } literal-2 ] }     )
   { condition-name } ( {              [ { - }           ] } ... )
                      ( { , literal-1                      }     )