VMS Help DECnet-Plus, DCL Commands, SET, HOST, /VTP, Qualifiers, /COMMAND  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /COMMAND=command-character

    Selects the command character. The command character is used to
    access VT command mode by pressing Ctrl/command-character.

    The command character can be any ASCII character between @ and z.
    You cannot select a character currently defined as either the
    break character (see the description of the
    /BREAK=break-character qualifier) or the disconnect character
    (see the description of the /DISCONNECT=disconnect-character
    qualifier).

    The ASCII characters between @ and z include the alphabetic
    characters in both upper and lower cases, the "at" sign (@),
    the backslash (\), the right bracket (]), the circumflex (^),
    the underscore (_), and the grave accent (`).

    By default, the command character is the "at" sign (@).