The ACCESS MODE clause specifies the order of access for a file's
  records.

1  –  1format_sequential

  The ACCESS MODE clause specifies the order of access for a file's
  records.

  Format 1 - Sequential File

   [ ACCESS MODE IS ] SEQUENTIAL

2  –  2format_relative

  The ACCESS MODE clause specifies the order of access for a file's
  records.

  Format 2 - Relative File

                      {  SEQUENTIAL [ RELATIVE KEY IS rel-key ] }
   [ ACCESS MODE IS ] { { RANDOM  }   RELATIVE KEY IS rel-key   }
                      { { DYNAMIC }                             }

2.1  –  rel-key

  is the file's RELATIVE KEY data item.

3  –  3format_indexed

  The ACCESS MODE clause specifies the order of access for a file's
  records.

  Format 3 - Indexed File

                      { SEQUENTIAL }
   [ ACCESS MODE IS ] { RANDOM     }
                      { DYNAMIC    }