To reproduce: radCheckedDropDownList1.CheckedDropDownListElement.AutoCompleteMode = AutoCompleteMode.SuggestAppend; radCheckedDropDownList1.CheckedDropDownListElement.AutoCompleteSuggest.SuggestMode = SuggestMode.Contains;
RadCheckedDropDown list supports Append, Suggest and SuggestAppend modes. Append and SuggestAppend are using StartsWith for its search, while Suggest uses Contains, just like in MS Outlook. The API mentioned should not be publicly exposed (it is derived from the underlying control and is not applicable here) and we will hide it with our next release. The documentation will also be updated accordingly.