The full command-line option is /[NO]DIAGNOSTIC[=file-spec].

    Controls whether diagnostics are created and stored in the
    specified optional file. If a file specification is not supplied,
    the assembler creates a diagnostic file using the same name
    as the source file. For example, if you use a source file
    named XXX.M64, the assembler creates a diagnostic file named
    XXX.DIA. You can use the diagnostic file with other Digital
    layered products including, but not limited to, the DEC Language-
    Sensitive Editor (LSE).

    The default qualifier is /NODIAGNOSTIC.