VMS Help Sys Parameters, WRITABLESYS  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    WRITABLESYS controls whether system code is writable. This
    parameter is set (value of 1) for debugging purposes only.

    This special parameter is used by Compaq and is subject to
    change. Do not change this parameter unless Compaq recommends
    that you do so.