destination-string

    OpenVMS usage:char_string
    type:         character string
    access:       write only
    mechanism:    by descriptor

    Destination string to which STR$APPEND appends the source string.
    The destination-string argument is the address of a descriptor
    pointing to the destination string. This destination string
    must be dynamic or varying length. The maximum length of the
    destination string for a 32-bit descriptor is 2**16-1, or 65,535,
    bytes.

    On Alpha systems, the maximum length of the destination string
    for all 64-bit descriptor classes, except varying-length strings,
    is 2**64-1 bytes. The maximum length of a varying-length string
    is 2**16-1 for both 32-bit and 64-bit descriptors.

 source-string

    OpenVMS usage:char_string
    type:         character string
    access:       read only
    mechanism:    by descriptor

    Source string that STR$APPEND appends to the end of the
    destination string. The source-string argument is the address
    of a descriptor pointing to this source string.