/BASE_SYSTEM=(base-system-name[,...])

    Selects software products whose base system matches the one
    specified. The base system name identifies both a hardware
    platform and an operating system. Standard names are:

    Name          Description

    AXPVMS        Denotes an OpenVMS Alpha product.
    I64VMS        Denotes an OpenVMS I64 product.
    VAXVMS        Denotes an OpenVMS VAX product.
    VMS           Indicates a product that can be installed on more
                  than one OpenVMS platform.

    Parentheses (())  are optional only when you specify a single
    base system name. They are required when you specify multiple
    base system names.