/sys$common/syshlp/helplib.hlb EDIT, /TPU, /MODIFY  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  /MODIFY    (default)
  /NOMODIFY

  Determines whether you can modify the main (or first) buffer.  If you
  specify multiple input files on the EDIT/TPU command line, this
  qualifier applies to each buffer.  Does not affect other buffers you
  create during the editing session.

  By default, DECTPU lets the buffer be modified---you can edit text in
  the buffer, and exiting writes out the buffer to a file, if the
  buffer has been modified (unless you used /NOWRITE or /READ_ONLY).
  Use /NOMODIFY to view a file without making any changes.  You can
  then use cursor-movement commands but cannot change the text.  If you
  do not specify /MODIFY or /NOMODIFY, the application is expected to
  determine the default behavior.  For EVE, the main buffer is made
  modifiable (as well as buffers initially created from your input
  files).

  For EVE, using /READ_ONLY or /NOWRITE makes the buffer unmodifiable
  unless you also use /MODIFY.  For example, the following command
  edits a file named PRACTICE.TXT, making the buffer read-only and
  making it modifiable, so you can practice editing or test procedures
  without writing a file:

     $ EDIT/TPU practice.txt /READ_ONLY /MODIFY

  In EVE, the status line shows whether the buffer is unmodifiable.  If
  the buffer is modifiable, the status line shows the mode (insert or
  overstrike).  You can change the modification attribute of the buffer
  during your editing session by using the SET BUFFER command.