The following script returns STD_ACD status:

if (Event.Fields['Engine.STD.EmployeeInsured.ACDStatus'] != null) {

    if (Event.Fields['Engine.STD.EmployeeInsured.ACDStatus'] == 1)

        Event.Fields['STD_ACD_Status'] = "A";

    if (Event.Fields['Engine.STD.EmployeeInsured.ACDStatus'] == 2)

        Event.Fields['STD_ACD_Status'] = "C";

    if (Event.Fields['Engine.STD.EmployeeInsured.ACDStatus'] == 3)

        Event.Fields['STD_ACD_Status'] = "D";

}