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

       /COMMAND=command-character

    Selects the command character. Use the command character to
    access DTE command mode by pressing Ctrl/command-character.

    The command character can be any ASCII character between @ and z,
    except C, M, Q, S, Y, or the left bracket ([).  You cannot select
    a character currently defined as either the break character (see
    the description of the /BREAK=break-character qualifier) or the
    escape character (see the description of the /ESCAPE=escape-
    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 (@).