VMS Help PASCAL, Input Output, General, OPEN, User Action  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  The 'user_action' is the name of  a  user-written  routine  that
  Compaq Pascal calls to open the file (instead of allowing Compaq
  Pascal to open the file with the OPEN procedure).  You can use a
  user-action  routine to open the file using environment-specific
  capabilities of the I/O system underlying Compaq Pascal.