To set the tooltip for each value selected from Drop Down list - means value is selected in drop down list and the mouse over on the field (i thought this way, if its not correct ignore the same.
For the above case check the selected drop down value on change event and set the desired text as tooltip.
For example
if(xfa.event.newText == "One")
this.assist.toolTip.value = "You Selected Choice " +xfa.event.newText;
Thanks,
Raghu.