This attribute stores the address of the icclass object's target object.
Whenever the icclass object receives an OM_NOTIFY or OM_UPDATE message, it
forwards that message to its target in the form of an OM_UPDATE message.
If the icclass object has an attribute mapping list (see the ICA_MAP
attribute below), it also maps the OM_NOTIFY/OM_UPDATE message's attribute
IDs to new ones before forwarding the message.

If the value of ICA_TARGET is ICTARGET_IDCMP, the icclass object sends an
IDCMP_IDCMPUPDATE IntuiMessage to its window instead of forwarding an
OM_UPDATE message.  See the rootclass description of OM_UPDATE for more
information.