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 4 -

   SET { { switch-name } ... TO { ON  } } ...
       {                        { OFF } }

1  –  switch-name

  is the name of an external switch defined in the SPECIAL-NAMES
  paragraph.