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

  /[NO]DIAGNOSTICS[=file-spec]    D=/NODIAGNOSTICS

  Controls whether the compiler produces a file containing compiler
  messages and diagnostic information.

  If you omit the file specification, the diagnostics file has the
  name of your source file and a file type of DIA.

  The diagnostics file is reserved for use with Compaq layered
  products such as the DEC Language Sensitive Editor (LSE).

  See also HELP on /SHOW=DATA_DEPENDENCES, which affects the .DIA
  output.