This attribute stores the address of the gadget's target object.  Whenever
the gadget receives an OM_NOTIFY message, it sends an OM_UPDATE message to
its target.  If the gadget has an attribute mapping list (see the ICA_MAP
attribute below), it also maps the IDs from the OM_NOTIFY message.

If the value of ICA_TARGET is ICTARGET_IDCMP, the gadget sends an
IDCMP_IDCMPUPDATE IntuiMessage to its window.  See the rootclass
description of OM_UPDATE for more information.