@Jason_Woods is correct. Pull all the UD codes along with UD character (sub group)
Then apply a local filter in the customization
(OnDropdown)
private void epiComboC2_BeforeDropDown(object sender, System.ComponentModel.CancelEventArgs args)
{
// ** Place Event Handling Code Here **
EpiDataView edv = oTrans.Factory("Contact");
epiComboC2.Rows.ColumnFilters["CodeID"].FilterConditions.Clear();
epiComboC2.Rows.ColumnFilters["CodeID"].FilterConditions.Add(Infragistics.Win.UltraWinGrid.FilterComparisionOperator.StartsWith,edv.dataView[edv.Row]["ShortChar03"]);
}