6.3.1 Opening and Closing a Terminal-Format File
  You use the OPEN statement to create a file, or to gain access
  to an existing file. If you do not specify either FOR INPUT or
  FOR OUTPUT in the OPEN statement, BASIC tries to open
  an existing file. If the file does not exist, BASIC creates a new
  one.

  The channel specification lets you associate a number with
  the file for as long as the file is open. All I/O operations to or
  from the file use this number.

  When you are finished accessing a file, you close it with the
  CLOSE statement.