If a file description entry has a FILE STATUS clause, a value is placed
  in the two-character FILE STATUS data item during execution of a CLOSE,
  DELETE, OPEN, READ, REWRITE, START, UNLOCK, or WRITE statement.  Two
  "keys" combine to form this value.  Status Key 1 occupies the leftmost
  character position in the item and Status Key 2 occupies the rightmost
  position.  In combination, the keys indicate the status of the
  input-output operation.