string

    OpenVMS usage:char_string
    type:         character string
    access:       read only
    mechanism:    by descriptor
    Command line. The string argument is the address of a descriptor
    of a DECTPU command.

 fileio

    OpenVMS usage:vector_longword_unsigned
    type:         bound procedure value
    access:       read only
    mechanism:    by descriptor
    File I/O routine. The fileio argument is the address of a
    descriptor of a file I/O routine.

 call_user

    OpenVMS usage:vector_longword_unsigned
    type:         bound procedure value
    access:       read only
    mechanism:    by descriptor
    Call-user routine. The call_user argument is the address of a
    descriptor of a call-user routine.