VMS Help SET, RMS_DEFAULT, Qualifiers, /PROLOG  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /PROLOG=n

    Specifies a default prolog level for indexed files where
    acceptable values for n are 0, 2, or 3. If 0 (default) is
    specified, RMS sets an appropriate prolog level.